(covers information from several alternate timelines)
Qo'noS, alternatively spelled as Q'onoS, also known as Kling or the Klingon homeworld, and transliterated to Kronos in English, was an inhabited class M planet in the Qo'noS system, the homeworld of the warp-capable Klingon species, and the capital planet of the Klingon Empire. (ENT: "Broken Bow", "The Expanse", "In a Mirror, Darkly, Part II" display graphic; Star Trek VI: The Undiscovered Country; TNG: "Sins of the Father", "Redemption"; DS9: "The House of Quark"; Star Trek Into Darkness; DIS: "Magic to Make the Sanest Man Go Mad")
The exact coordinates of the planet were 43.89.26.05 in grid 09, quad 68, block 27, sector 70. (Star Trek Into Darkness, display graphics) Qo'noS was about four days away from Sol at warp 4.5 (ENT: "Broken Bow")
Described as a "planet of caves", the planet was composed of a subterranean series of volcanoes, which lay dormant for centuries, to the point of being considered extinct, before becoming alive again in the 2250s. Some caves were of substantial size, including caves not connected to the volcano system. (DIS: "The War Without, The War Within", "Will You Take My Hand?")
The planet's upper atmosphere was dense, preventing Starfleet from conducting long-range topographical and energy analysis, at least until the mid-24th century. As such, the Federation went into its first war with the Klingons with Qo'nos considered uncharted territory. (DIS: "The War Without, The War Within", "Will You Take My Hand?")
- See also: Klingon history
As the homeworld of the Klingons and the capital world of the Klingon Empire, Qo'noS housed the Klingon High Council and other important institutions. As claimed by Quark, warp capability was achieved sometime after 1947 and warp 6 had been achieved sometime before 2152. (TNG: "Rightful Heir"; DS9: "Little Green Men"; VOY: "Day of Honor"; ENT: "Judgment")
Humans first visited Qo'noS in 2151, when, following first contact with the Klingons, the starship Enterprise, Earth's first deep space exploratory vessel, reached the planet using Vulcan star charts. Enterprise visited the planet again during the next decade, after which it was not visited by Starfleet for nearly a century. (ENT: "Broken Bow"; DIS: "The War Without, The War Within")
During the 22nd century, the Vulcans acquired a black market survey of the planet. It remained some of the only intelligence Starfleet had on the planet even a century later. (DIS: "Will You Take My Hand?")
Despite the Klingons' xenophobic views, the Orions held land on Qo'noS, which they used as an "embassy outpost." The outpost had a hefty non-Klingon population, dominated by Orions, and even the presence of Humans was not significantly questioned. (DIS: "Will You Take My Hand?")
In 2293, the ozone layer on Qo'noS was severely damaged when Praxis, a key energy-production facility for the Klingon Empire, exploded after an accident involving over-mining, its destruction sending out a subspace shock wave. This event forced planning for the evacuation of Qo'noS and led to the signing of the First Khitomer Accord with the Federation. (Star Trek VI: The Undiscovered Country) The location of Kling in the Milky Way Galaxy was labeled in a star chart that was in Captain James T. Kirk's quarters aboard the USS Enterprise-A. (Star Trek VI: The Undiscovered Country, okudagram)
In the mirror universe, Qo'noS was turned into a blackened ball of dust after Terran emperor Philippa Georgiou detonated a hydro bomb in the vents of the planet's volcanic system. This triggered a phreatic eruption and rendered the planet uninhabitable within weeks. The attack didn't result in the extinction of the Klingons, although Terran ships did fire on ships trying to escape. (DIS: "The War Without, The War Within", "Will You Take My Hand?") Subsequently, Georgiou held the title Dominus of Qo'noS. (DIS: "Vaulting Ambition")
In the alternate reality, Praxis had been fragmented by 2259, when Khan Noonien Singh took refuge on Qo'noS after attacking Starfleet Headquarters. The USS Enterprise was ordered to execute him using advanced long-range torpedoes, but Captain James T. Kirk elected to descend to Qo'noS in a K'normian trading ship to apprehend Khan instead. After an altercation with a Klingon patrol, Khan surrendered to Kirk once he surmised that his crew were located within the torpedoes. (Star Trek Into Darkness)
Confederation of Earth timeline
In an alternate timeline where the fascist Confederation of Earth existed in place of United Earth and subsequently the United Federation of Planets, General Jean-Luc Picard decimated Qo'noS with a bioengineered virus. (PIC: "Penance")
- Bar'or Lhess
- First City (capital city)
- Ha'r Kling Jaa
- H'Vek Tar
- Lhess Taal Dax
- L'vln Lak Raal
- Lyn'Knor Tak
- Mor'em Cha
- Tong Vey
- T'Val K'rang
Flora and fauna
- Glob fly
- Grint hound
- Grishnar cat
- Jackal mastiff
- Monster dog
- Kolar beast
- Night-Blooming Throgni
- Saber bear
- Serpent of Xol
- Serpent worm
Establishing the homeworld
During development of the aborted TV series Star Trek: Phase II, John Meredyth Lucas wrote a two-parter, called "Kitumba", which would have featured the Klingon homeworld. The planet was described as being only marginally larger than Earth. Because of the Klingons' belief in their invincibility, the planet would have had no defense systems. The Klingon warlord, who presided over political and military decisions, resided here. (Star Trek: Phase II - The Making of the Lost Series)
The Klingon homeworld was also to have featured in the second Star Trek film, as planned by Gene Roddenberry. Susan Sackett wrote that the movie would feature not only an extensive insight into the Klingons' way of life but additionally "an inside look at their home planet." (The Making of Star Trek: The Motion Picture, p. 215) This also, however, did not enter development beyond the conceptual stage.
In an interview conducted about midway through TNG Season 3, Worf actor Michael Dorn was asked if he would like the Klingon homeworld to be featured in an episode. He replied, "That's an easy way out." (Star Trek: The Official Fan Club Magazine issue 72, p. 5)
The Klingon homeworld first appeared later in TNG's third season, in the episode "Sins of the Father". Both "Our Brother's Keeper” by Beth Woods and "Brother to Dragons" by Drew Deighan, scripts which inspired "Sins of the Father", included the Klingon homeworld; "Brother to Dragons" was partly set on the planet and "Our Brother's Keeper" referenced it. ("Sins of the Father" audio commentary, TNG Season 3 Blu-ray) Writer Ronald D. Moore was pleased to be asked to script "Sins of the Father" precisely because it was to be the first installment to show the planet. (Star Trek: Communicator issue 114, p. 58) Director Les Landau similarly relished the task of representing the planet, later enthusing, "Here was an opportunity to explore the Klingon world in depth from beginning to end." (Captains' Logs: The Unauthorized Complete Trek Voyages, p. 194)
At the outset of writing "Sins of the Father", Ron Moore provided Co-Executive Producer Michael Piller with an explanatory two-page memo about the Klingons which included the statement, "The Klingon homeworld is a world of extremes. The tilt of the planet's axis results in wild seasonal changes and there is a great deal of volcanic activity. There's a single land mass and an enormous, turbulent ocean. The race we know as Klingon was born on this harsh and brutal planet." Presumably referring to the same planet, the memo also noted, "The Klingon Empire sprang from a single, relatively poor planet in a modest star system." ("Sins of the Father" audio commentary, TNG Season 3 Blu-ray) The final draft script for "Sins of the Father" echoed some of those ideas, stating, "The planet has a single large land mass, and the world is dominated by a vast ocean. The tilt of the planet's axis has created wild seasonal changes and the atmosphere should appear turbulent and showing extremes of both warm and frigid weather on the planet." 
Upon establishing Qo'noS in "Sins of the Father", the creative team opted to give the Klingon homeworld a murky green sky and make the planet, as seen from space, appear primarily green. By way of explanation, Visual Effects Producer Dan Curry stated, "I guess [it was] my homage to [Altair IV in] Forbidden Planet, which had a green atmosphere, and we just felt that there would be a degree of noxiousness on the Klingon homeworld." ("Sins of the Father" audio commentary, TNG Season 3 Blu-ray)
For "Sins of the Father", Syd Dutton of Illusion Arts was assigned to create a matte painting of the Great Hall and surrounding First City. (Star Trek Encyclopedia 1st ed., p. 118; 3rd ed., p. 178) When Dutton and Dan Curry were planning this painting, Curry took inspiration from personal experience of having spent much of his youth in Asia, primarily Thailand, Laos, and Nepal. Said Curry, "I wanted the Klingon architecture to kind of be an amalgam of those architectural styles […] Syd immediately got what I was talking about and Syd's first sketch became the final sketch, and Syd would do a sketch on brown wrapping paper in oils and then […] I'd talk to [Executive Producer] Rick Berman and [Co-Producer] Peter Lauritson about it and they would go with it, so we went ahead with the final version." The illustration was ultimately still an oil painting, though painted on Masonite, and measured about four feet wide. Purposefully making the painting very wide enabled focus on different portions of the illustration to represent different parts of the city.
The matte painting of Qo'noS designed for "Sins of the Father" had some additional elements composited together with it, such as rear-projected footage of people walking around between columns in the exterior of the Great Hall. The painting was also to show the First City amid a lightning storm. "The lightning was composited on an optical printer, not a digital composite," clarified Dan Curry. "Syd [Dutton] would then paint different angles of light for the reflections, so it wasn't just a flash frame but you felt that the lightning was directional." The painting was composited by Dutton's partner at Illusion Arts, Bill Taylor. ("Sins of the Father" audio commentary, TNG Season 3 Blu-ray)
Also for "Sins of the Father", Production Designer Richard James and Set Decorator James Mees had to devise the creation of sets for the Klingon homeworld. Les Landau explained, "[They] had to visualize [the planet] in terms of set design and set decoration. Additionally […] Marvin Rush, the cameraman, had to conceptually find a visual representation of what the Klingon world was all about [....] There were long detailed conversations about how, conceptually, we should deal with them [i.e. the challenges of showing the planet in live-action]." (Captains' Logs: The Unauthorized Complete Trek Voyages, p. 194)
Multiple sets and set pieces used to represent Qo'noS in "Sins of the Father" were reused from previous Star Trek productions. Ron Moore commented, "We didn't build anything for 'Sins' by itself, I don't think." Circular arches on the planet were first made for TNG: "The Neutral Zone", in which they were encompassed in the interior of the cryosatellite featured in that episode. A street in the First City was made from pre-existing set pieces which were integrated especially for "Sins of the Father". Even the Great Hall was substantially redressed from having been used as the laboratory aboard the Tanuga IV research station, shown in "A Matter of Perspective". ("Sins of the Father" audio commentary, TNG Season 3 Blu-ray)
The sets created for Qo'noS in "Sins of the Father" were hugely successful. "Ultimately, Rick Berman gave the final approval for each and every one of the ideas and details," stated Les Landau, "and we showed a world that was heretofore never seen before, and which the audience craves to see more of." As such, Landau was extremely pleased with the amount of effort that James, Mees, and Rush went to in helping depict the planet. (Captains' Logs: The Unauthorized Complete Trek Voyages, p. 194) The interior design of the Klingon Great Hall and other sets in "Sins of the Father" actually earned James and Mees the Emmy Award for Best Art Direction. Upon viewing "Sins of the Father" many years after having written it, Ron Moore was amazed by the extensiveness of the sets for the Klingon homeworld, shortly thereafter remarking, "I was struck by how many sets we had, and this was one of the bigger pieces we did on the show, because we didn't normally have this many sets on an alien world." ("Sins of the Father" audio commentary, TNG Season 3 Blu-ray)
According to Ron Moore, the entirety of "What Dreams May Come" – a story by Drew Deighan that gradually evolved into becoming "Reunion" – was set in orbit above the Klingon homeworld, involving the Enterprise-D and two Klingon battle cruisers. Production changes prevented the planet appearing in "Reunion" at all, though. "You know, there were lots of sequences down on the Klingon homeworld," said Moore. "Those got cut very early on." In early drafts of the outing, the Klingon homeworld was to have been the location where Duras went, after killing K'Ehleyr, pursued there by Worf (who ends up killing Duras) and Riker. ("Reunion" audio commentary, TNG Season 4 Blu-ray) However, this setting was clearly altered to Duras' Klingon Bird-of-Prey, the IKS Vorn.
For "Redemption II", a matte painting of the Klingon homeworld on fire, during the Klingon Civil War, was done by Syd Dutton and Robert Stromberg at Illusion Arts. (Cinefantastique, Vol. 23, No. 2/3, p. 35)
Much consideration was given to whether more of the Klingon homeworld would appear in Star Trek VI: The Undiscovered Country than does feature into that film, showing more than just a Klingon courtroom. Screenwriter Mark Rosenthal explained, "What happened was that they felt in terms of budget, recreating the entire planet would be impossible, so it became this prison concept [on Rura Penthe]. The original idea was to go to the actual capital city. I still think it was a better idea." (The Making of the Trek Films, 3rd ed., p. 100)
Ron Moore was happy that DS9 Season 3 installment "The House of Quark" provided an opportunity to revisit Qo'noS, later reminiscing, "I was glad to go to the Klingon homeworld for a change." (Cinefantastique, Vol. 27, No. 4/5, p. 90) Moore also non-canonically imagined that Qo'noS was visited by Worf in the interim between the destruction of the Enterprise in Star Trek Generations and his arrival on Deep Space 9 in DS9 Season 4 premiere "The Way of the Warrior". (Star Trek: Communicator issue 104, pp. 60-61)
In an ultimately discarded line of dialogue from the first draft script of DS9: "Sons and Daughters", General Martok, in reference to bloodwine, commented, "I favor the autumn crop from the northern district on Qo'noS."
In Ron Moore's first draft of DS9's final Klingon episode, "Tacking Into the Wind", Gowron returned to Qo'noS at the end of the installment. The planet isn't depicted in such a way in the episode's final version, though, as Gowron is instead killed aboard Deep Space 9. (Star Trek: Deep Space Nine Companion (p. 691); Cinefantastique, Vol. 32, Nos. 4/5, p. 74)
Even though the regular characters on Star Trek: Voyager included half-Klingon B'Elanna Torres, the premise of the series meant the show couldn't regularly feature Qo'noS as a setting. Even references to it were strictly governed over; there was a policy that dictated the Klingon homeworld couldn't be referred to as the Klingon "home planet". (Star Trek: Voyager - A Vision of the Future, pp. 164 & 323)
Kronos was mentioned in the series bible for Star Trek: Enterprise. In that document, it was stated that, if Hoshi Sato was given ten minutes with a Klingon, she'd soon be chatting about the weather on Kronos.
The script of ENT: "Broken Bow" specified that, in that episode, a visual effect of the Klingon homeworld was to represent the planet during one of its days. The document also detailed the view of the planet thus; "The gothic towers of the Klingon High Council Chamber rise above the smoggy yellow haze of the teeming capital city."  Dan Curry recalled, "My immediate reaction [to the script] was, 'Let's contact Syd Dutton at Illusion Arts.'" ("NX-01 File 02", ENT Season 1 special features) Hence, Illusion Arts went on to create the episode's matte painting of the Klingon capital. (Star Trek: The Magazine Volume 2, Issue 10, p. 30) Continued Curry, "We looked at reference photographs and we found a photograph of some place in Morocco or North Africa that was in Syd's library and we loosely based the terrain on it." Since Dutton was very busy at the time, however, he brought in another matte painter, Bobby Scifo, who did the shot of Qo'noS that was finally used for "Broken Bow". It was decided that this representation of the Klingon homeworld would feature a large canyon. "Some of the staff at Illusion Arts, I think it was John Wong, built a very rough, large-scale model, maybe ten feet across, of this canyon with Klingons in it," explained Curry. The miniature allowed the team to view the buildings from different perspectives, knowledge which they then used to decide how the final shot would look, incorporating the same buildings from the model. ("NX-01 File 02", ENT Season 1 special features)
During the filming of ENT: "Unexpected", a green screen, on Paramount Stage 9, stood in for a holographic view of Kronos, a perspective from "a hilltop overlooking the great city below," as stated in the episode's final draft script. Next, the green screen was replaced with a CGI depiction of that view. (Information from call sheets)
Brannon Braga opted for the Klingon homeworld not to appear as the location of a Klingon courtroom in the Enterprise installment "Judgment", which was instead subsequently intended to be on Narendra III. This was because the homeworld had been shown earlier in the series and Enterprise was conceived to generally be, at the point when "Judgment" was due to air, moving further out into space. ("Judgment" audio commentary and podcast)
The holographic view of Kronos from "Unexpected" was also included in an amended complaint of copyright infringement in the case of Paramount Pictures and CBS Studios Inc. vs. Axanar Productions and Alec Peters (the court document was filed on 11 March 2016). That document also stated that Qo'noS had first appeared in "Sins of the Father" and had subsequently "played a large role" in Star Trek VI: The Undiscovered Country.
Star Trek: Discovery Production Designer Tamara Deverell noted, "We decided to go to Qo'noS, but not just the average Klingon Qo'noS." ("Designing Discovery", DIS Season 1 DVD & Blu-ray special features) Indeed, when Qo'noS reappeared in DIS Season 1 finale "Will You Take My Hand?", the design of the planet, particularly the look of a black market Orion outpost in the episode, reflected the fact that the creators of DIS were putting more emphasis on exploring the emotional depths of the combative Klingon people. (For more information, see Orion outpost.) 
The alternate reality
For the planet's appearance in Star Trek Into Darkness, the filmmakers were inspired by Pripyat, the city abandoned due to the Chernobyl disaster that inspired Praxis in The Undiscovered Country. Roberto Orci commented about the planet, "Maybe its civilization was built on top of the old version of the Klingon civilization, so Scott Chambliss did an amazing job of putting the new on top of, sort of, ruins of a city that may have gone through a Praxis-like event."  Chambliss himself explained, "At first, Qo'noS was going to be a super-industrialized place, where the off-product of their industry was so toxic, they basically turned the planet itself into an utter waste zone. In doing that, they altered the environment so much it became a non-stop storming planet. There was only lightning, storm, and crazy wind in the sky. You couldn't even walk the surface of the planet, because it was all oceans of toxicity. They had burrowed down into the planet, as well as built their city on top. They kept building higher and higher because the meth level got higher and higher."
The concept of the highly polluted Klingon homeworld went on to influence the design of the place. "When it came down to dealing with it," recalled Scott Chambliss, "the [battle] sequence wasn't fully fleshed out in the beginning, which is normal [....] Ultimately, that took us to a place of trying all different kinds of environmental pieces of this overall picture of toxified Qo'noS [....] I think I did 14 different versions of the set. Each one had a different specificity of what it was, and why it was the way it was. Nothing really landed with J.J. [Abrams.] It finally got to the point where I needed to do something a lot more abstract." Chambliss also remarked that the area of Qo'noS featured in the battle sequence was "probably a factory" and possibly had been "a munitions factory" or "a place for building spacecraft." (Star Trek Magazine issue 172, p. 67)
Scott Chambliss took classical influence for the columns in this region. "The quadrant of the city where the action takes place is sort of an interpretation of the Roman Colosseum, the warrens below the Colosseum floor," he said. 
The idea of designing the set in an extremely abstract way led Scott Chambliss to propose a particular feature of the site, which he described as a "big, glowing, lensy thing in the background, the big halo." This element was based on the sculptural installation The Weather Project. "That was the thing I kept staring at, and seeing the scale of a Human being in front of it," he reflected, "and thought, 'This is so cool. Maybe this is a way into what we're doing, playing with a massive sunburst or light source, or whatever the hell it was going to be, against our villain, who is supposed to be the baddest guy on Earth.' But the epic scale of that glow even makes this huge bad guy look tiny.'" (Star Trek Magazine issue 172, p. 67)
Following the set's construction, the rest of the planet was fleshed out. At Pixomondo, the look of the world was designed by CG Supervisor Enrico Damm and Compositing Supervisor Dan Cobbett. Remembered Pixomondo Visual Effects Supervisor Ben Grossmann, "The way J.J. turned it over to us was as a moody and atmospheric environment [....] For the larger planet and the flying through it, we tried to take as many design cues from the partial, destroyed set that was there as far as textures and design angles, and extrapolate from that what a city, planet, and building would look like.” These aspects became a combination of industrial areas and ten-mile-high building spires existing in the dense, polluted air. Pixomondo also developed a toxic atmosphere from which the D4-class ships could appear. Grossmann added, “We took reference from the Burj Khalifa in Dubai and from really tall and spire-y buildings [....] What I liked was when you see pictures photographed from up there, the buildings are so tall that they peek through the clouds and you have no real sense of how far you are away from the ground. That gave us the idea to build the planet Kronos in atmospheric layers. We tried to imagine what a planet like Jupiter would be like underneath all these layers of the atmosphere."
The upper levels of Qo'noS were based on thinking that, in the upper atmosphere of a Jupiter-like planet, there might be occasional clarity amongst the thick clouds. “As we start to descend down through the atmosphere," stated Ben Grossmann, "we had the idea to almost give it an underwater feel. We figured that with that much humidity and moisture to create a toxic cloud, you’d imagine there’s a lot of toxic sludge around. We ran a bunch of waterfall simulations and we allowed pretty high amounts of accumulated condensation to create these toxic waterfalls.”
Lower in the planet's atmosphere, the lighting shifted to seem akin to an underwater cave, with shafts of light scattering in the atmosphere and illuminating the environment. “We realized the most effective way to nail this look and make it feel real was by rendering as much as possible the volumes of atmosphere all in one,” Ben Grossmann recounted. “Normally you just render lighting passes and let the compers dial in the balance [....] But when you’re shooting everything through a really heavy atmosphere, that technique doesn’t work anymore. So what you have to do is render the effects volumes properly lit from within and calculating the qualities of the light as they move through varying densities of the atmosphere. Needless to say, that was quite a render hog but gave us the look we were going for in the end.”
The fluid simulations were done in Naiad, whereas volumetric effects – such as atmospheric swirls from wing tips – were rendered in FumeFX. Ben Grossmann concluded, "To help give a sense of scale to these volumes we ran blowing air debris and bits of ash and detritus as fluid sims rendered out with Thinking Particles. And we used Krakatoa too […] Buildings [were] rendered in V-Ray." Minor CGI was involved even in the area used for the battle sequence, such as tattered pieces of fabric which were included as ground elements and were created as cloth sims. 
Naming the homeworld
It took a long time before the Klingon homeworld received a definitive name on-screen. Ron Moore later remembered, "There was a matter of great debate, within the [TNG] staff, about naming the Klingon homeworld. It was one of these tedious discussions that went on ad nauseum, far beyond its actual importance, because writers would just sort of name the Klingon homeworld and then everyone would hate it […] Memos flew around about this, and we'd sit and we would write all these Klingon names […] to try and come up with something that sounded decent and interesting and strong, for their homeworld." ("Sins of the Father" audio commentary, TNG Season 3 Blu-ray) During the making of "Sins of the Father", linguist Marc Okrand was invited to name the planet. "The Homeworld was a big deal," Okrand recalled. "I actually came up with about six names, some sounded good, some actually meant something. Paramount gave them to Gene Roddenberry and he picked his favorite. I had no further connection with it until I saw the episode and all they said was 'we're going to the Homeworld.'" (Star Trek: Communicator issue 114, p. 58) Indeed, the writing staff went through a period, between considering names, in which they planned to refer to the planet merely as "the homeworld." Commented Ron Moore, "Then that got awkward, 'cos you were constantly saying, 'on the homeworld,' and, 'Where's the homeworld?' [and] 'Set course for the homeworld.' And so we started trying to come up with a name." ("Sins of the Father" audio commentary, TNG Season 3 Blu-ray)
Several names for the planet have been established on screen and in reference works. An NBC press release which was issued on 3 January 1968 and pertained to "A Private Little War", for instance, referred to the Klingon planet's name simply as "Klingon." (These Are the Voyages: TOS Season Two) As another example, the Star Trek: Phase II episode "Kitumba" referred to the Klingon homeworld as "Ultar". (Star Trek: Phase II - The Making of the Lost Series) According to Ron Moore, Marc Okrand suggested naming the planet either "tlhIngan" (the unofficial Klingonese name for the Klingon species) or something similar, which Moore considered "almost unpronounceable." ("Sins of the Father" audio commentary, TNG Season 3 Blu-ray)
- Kronos: The Klingon homeworld remained canonically unnamed until Star Trek VI: The Undiscovered Country, in which it was identified as "Kronos", spelled that way in the film's script.  It was Marc Okrand who originally contributed this name, though he took a long while to come up with it. The writing staff of TNG were reluctant to adopt the moniker. Ron Moore stated, "Rick [Berman] or Michael [Piller] or somebody didn't want to use it in the show, like that name was not to be used, and we were just gonna call it the homeworld […] Eventually, I think, it came back around to Kronos, to the point where everyone had forgotten why they objected to it in the first place." ("Sins of the Father" audio commentary, TNG Season 3 Blu-ray) The spelling "Kronos" was also used in most of the scripts of Star Trek: Deep Space Nine episodes.  It was also used in the series bible for Star Trek: Enterprise and the scripts for the ENT episodes "Broken Bow" , "Unexpected", "Silent Enemy", "Bounty", "Home" (the latter of which can be seen in the ENT Season 4 Blu-ray), and "The Augments". The same spelling was also used in the call sheets and shooting schedule for "Unexpected". According to the pronunciation guide in the script of DS9: "Tears of the Prophets", "KRO-nos" is the phonetic spelling of "Kronos".  The pronunciation guide in the "Unexpected" script notated its pronunciation the same way, as did the script pronunciation guide for "Silent Enemy". The spelling "Kronos" was first seen on-screen in Star Trek Into Darkness.
- Qo'noS: According to the Star Trek Encyclopedia (3rd ed., p. 253) and Marc Okrand's reference book The Klingon Dictionary, "Kronos" is the pronunciation and Anglicized form of the Klingonese word "Qo'noS". According to the Klingonese transcription of the Klingon death chant from the script of "Tears of the Prophets", however, "Kronos" is the way the name is spelled in Klingonese as well.  The alternate spelling "QO'NOS" was sometimes used instead of "KRONOS" in closed-captioning, in the official DVD releases of Star Trek: Deep Space Nine episodes for example. On screen, the first time the spelling "QO'NOS" appeared was in Star Trek Into Darkness, displayed on the viewscreen of the Enterprise when Sulu sends a message to John Harrison.  The spelling Qo'noS was also exclusively used in the novelization of Star Trek Into Darkness and in the aforementioned court document pertaining to copyright infringement and Star Trek: Axanar.
- Q'onoS: The planet's name is consistently (I.e. twice) spelled "Q'onoS" on-screen in a personnel file for Hoshi Sato, which appears in ENT: "In a Mirror, Darkly, Part II".
- QonoS: In Klingonese dialogue from ENT: "Sleeping Dogs", the planet's name was spelled "QonoS" in the episode's final draft script.
- Chronos: "Chronos" was a more rare alternative spelling, used in the scripts of only two Star Trek: Deep Space Nine episodes. These were "Once More Unto the Breach"  and "Extreme Measures". 
- Kling: The original behind-the-scenes name for the planet was apparently "Kling", thought up by Gene Coon, who also originally conceived the Klingon species. D.C. Fontana stated, "Once we asked him, 'Where do Klingons come from? What's the name of their planet?' He said [laughing], 'Kling.'" (These Are the Voyages: TOS Season One) "Kling" was additionally used as an alternate name for the Klingon homeworld during the first season of Star Trek: The Next Generation. The name was first mentioned on-screen in TNG: "Heart of Glory" and depicted as a location on "The Explored Galaxy" star chart. At the time "Heart of Glory" was written, "Kling" was in fact intended as the name of the Klingon homeworld, but once the episode was filmed, it was realized that the name sounded pretty silly. (Star Trek Encyclopedia (4th ed., vol. 1, p. 426)) Despite this, either "Our Brother’s Keeper" or "Brother to Dragons" reused the name, with the other script using a similar alternative. Ron Moore noted, "I think one just called it 'Kling,' which felt bad, and the other called it 'Klinga,' which was even worse." ("Sins of the Father" audio commentary, TNG Season 3 Blu-ray) In the audio commentary for "Sins of the Father", Star Trek Encyclopedia co-writer Denise Okuda tells Ron Moore, "I'm glad you got away from 'Kling,'" and proceeds to mock the name with him. In the Star Trek Encyclopedia (3rd ed,. p. 243), "Kling" was referenced as a district or a city on the Klingon homeworld, apparently due to the name having been deemed silly following the making of "Heart of Glory". In dialogue deleted from "Heart of Glory", Worf said, "Could we speak English, sir? I am not comfortable in Kling." with the implication being that Kling was the official name for the Klingon Language, as well. . According to the reference book Star Trek: Star Charts (pp. 55 & 56), "Kling" is an alternative name for the homeworld, in common with "Kronos" and "Qo'noS". The nature of Kling was finally confirmed as an alternative name for Qo'noS during the first season of Star Trek: Discovery, of which several episodes, most notably "Magic to Make the Sanest Man Go Mad" showed a map based on the Star Trek: Star Charts. The word "Kling" also appears in the name of the Qo'noS city of Ha'r Kling Jaa and the "data point" on the planet named Vort Kling Tuuk.
- Klingonii, Klinzhai, Kazh: Reference works that predate Star Trek VI: The Undiscovered Country have offered several other names. According to the Star Fleet Medical Reference Manual (released in 1977), the Klingon homeworld was the planet "Klingonii", also known as "Epsilon Sagittarii B". According to Star Trek Maps' Chart B (released in 1980), the homeworld was the planet "Kazh" and the star it orbited was called "Klingon". In the reference book The Worlds of the Federation (pp. 45, 114, & 154), the homeworld was called both "Klinzhai" and "Kling". The name "Klinzhai" first appeared in the 1984 novel The Final Reflection. "Kling" and "Klinzhai" have also been used in the FASA RPG and various other novels, as well as in the Star Trek: Starfleet Command video game series, before the release of Star Trek VI.
Location of Qo'noS and the Empire
In dialogue, the Klingons have always been exclusively identified as an Alpha Quadrant race. The Klingon association with the Alpha Quadrant began to be established in Star Trek: The Next Generation. In "A Matter Of Honor", the Klingons are mentioned as having had a series of skirmishes with the Romulans along their shared border. In "Reunion", K'Ehleyr states that Klingon wars are rarely confined to the Empire. She suggests the current war might eventually spread to the neighboring star systems, and eventually to the Tholians and the Ferengi. All three were later established to be Alpha Quadrant powers, in such episodes as DS9: "The Search, Part II" and "Call to Arms" and VOY: "Q2".
Star Trek: Deep Space Nine episodes confirmed what had already been stated, additionally adding to it. In "The Die is Cast", the Dominion identify the Klingons as a threat originating from the Alpha Quadrant. "The Way of the Warrior" was the first episode that implied that the Klingons and the Cardassians share a border. In that installment, the Klingon Empire launches an attack against the Cardassian Union for the best interest of the Alpha Quadrant. In "Blaze of Glory", Captain Benjamin Sisko and Martok discuss a Dominion retaliation, saying that it would be not only against the Federation but also against the Klingons and the entire Alpha Quadrant. In "Tears of the Prophets", Sisko states that the Klingons, along with the Humans and the Romulans, have one goal: to drive the Dominion out of the Alpha Quadrant. In "When It Rains...", Chancellor Gowron envisions the Klingon Empire as the saviors of the Alpha Quadrant if they can drive the Dominion out of the quadrant.
The association between Qo'noS and the Alpha Quadrant was carried over to the dialogue in Star Trek: Voyager as well. In "Faces", Chief Engineer B'Elanna Torres boasts that Klingon females are renowned in the Alpha Quadrant for their physical prowess and voracious sexual appetites. In "Flashback", Captain Kathryn Janeway reminisces about what it would be like to live in Captain James T. Kirk's era, with the Alpha Quadrant still largely unexplored and Humans on the verge of war with the Klingons. In "Flesh and Blood", Chakotay identifies the Klingon bat'leth as an Alpha Quadrant weapon.
The close proximity of Qo'noS and Earth was established in the pilot episode "Broken Bow" of Star Trek: Enterprise. Qo'noS was located only four days away for a starship traveling at warp 4.5. "Two Days and Two Nights" established that this was at most only ninety light years away, as that was the farthest away from Earth any Human had ever gone, up to that point.
In Star Trek Into Darkness, Qo'noS' close proximity to Earth was further confirmed, as faster starships could easily reach the planet from the Sol system and return within a day. The film also includes a star chart display graphic of the Neutral Zone and the Klingon Empire seen in the offices of Admirals Pike and Marcus; while not in focus or legible in the film itself, the star chart graphic was labeled, in the lower corner, as a map of the Alpha Quadrant. All the vagueness about the homeworld was at least concluded in a targeted comm burst display graphic from the film, directly establishing the location of the planet Qo'noS to be in the Beta Quadrant. 
Before this, reference works that predated Star Trek: The Next Generation have depicted the location of the Klingon Empire and the homeworld in what has been, later on, established as the Alpha Quadrant. These include such works as the Star Trek Star Fleet Technical Manual (T0:02:07:00) and Star Trek Maps (Chart A). More recent reference works have, however, moved the Klingon Empire into the Beta Quadrant. They show that the Empire borders only the Romulan Star Empire and the United Federation of Planets. Despite the conflicting canonical dialogue references establishing the Ferengi and the Cardassians as neighbors of the Empire, they are depicted on the other side of the Federation, away from the Klingons in these graphics. Only one of these star chart graphics depicting the Klingons in the Beta Quadrant has made a brief on-screen appearance, as a display graphic on a PADD in Star Trek: Insurrection.   The information on the PADD, though, was completely illegible on screen. 
According to Star Trek: Star Charts (p. 55), the capital city for the uniglobal government and for the Klingon Empire was the First City of Qo'noS. The dominant species on the planet were the Klingons. Points of interest on this world were the Great Hall, Qam-Chee, Tong Vey, Quin'lat, Temple of G'boj, Kri'stak volcano, Lake of Lusor, and Caves of Kahless. The Klingons were warp-capable in 930 AD. The planet was on the front line during the brief Klingon Civil War of 2368. In 2378, there were 3.84 billion Klingons living on the planet. Star Trek: Star Charts also shows Qo'nos as clearly being in the Beta Quadrant (p. 55), in very close proximity to Theta Hydrae, which is approximately 113ly from Sol, not the 90ly stated in dialogue. A Star Charts-derived chart seen in "Magic to Make the Sanest Man Go Mad" corroborates this.
One theory (relayed by Richard Arnold but effectively disproved in the Star Trek: Enterprise two-parter "Affliction" and "Divergence") held that alternate hemispheres of the Klingon homeworld were responsible for the differences between Klingons with cranial ridges and those without. (Star Trek: Communicator issue 104, p. 12) Since a range of white and African-American performers played Klingons on Star Trek: The Next Generation, different hemispheres of the planet were indeed given associations for the contrasts in the make-up. Make-Up Supervisor Michael Westmore explained, "We settled on this strategy: all the natural brown-toned actors were Northern Klingons [....] All the fair-skinned actors were Southern Klingons." (Star Trek: Aliens & Artifacts, p. 68) A similar hypothesis was suggested by Brannon Braga; shortly prior to the premiere of Enterprise in 2001, he postulated, "Maybe there's a subspecies from a different continent on the Klingon homeworld."
In reply to a Klingon-loving fan criticizing John Colicos' pronunciations of Klingonese in DS9: "Blood Oath", Colicos claimed an obscure area of the planet was where his character of Kor was from. "I said, 'Oh, I come from a section of the Klingon planet |over the hills and far away – in the southern part, where it's warmer,'" the actor relayed. (The Official Star Trek: Deep Space Nine Magazine issue 15, p. 17)
The soundtrack of Star Trek Into Darkness references this planet; one composition is titled "The Kronos Wartet", which like every track by Michael Giacchino, is also a pun, in this case referencing the Kronos Quartet.
The short story "A Warrior's Path" in New Worlds, New Civilizations references Kling as a city on Qo'noS.