The D7 was introduced in 2257 by Chancellor L'Rell as she presented the ships's schematics to the Klingon High Council. She hoped that the design would enter mass production and be a symbol of Klingon unity embraced by all Houses. (DIS: "Point of Light")
One of the first D7-class vessels ferried Chancellor L'Rell to meet with the USS Discovery at Boreth. Several of these ships later fought the Section 31 armada in support of Discovery and USS Enterprise. (DIS: "Through the Valley of Shadows", "Such Sweet Sorrow, Part 2")
The D7 class served as the backbone of the fleet from the late 2250s through the 2270s, where they posed a serious threat to the security of the Federation and Starfleet. (TOS-R: "Errand of Mercy", "Elaan of Troyius", et al. TAS: "More Tribbles, More Troubles") Among the mission profiles designated for this class was that of scout ship. (TOS-R: "Friday's Child", "A Private Little War")
Interest in these vessels by the Federation continued to appear well into the 2370s, as data on this class was commonly found within numerous Starfleet files. (TNG: "The Naked Now", "The Last Outpost", "Conspiracy"; VOY: "Drone", "The Voyager Conspiracy")
At least one D7 remained operational until 2377, when it was used by a Klingon religious sect that left the Klingon Empire in the mid-23rd century on a multi-generational quest to the Delta Quadrant. This vessel was destroyed when the ship's commander, Kohlar, initiated a self-destruct. (VOY: "Prophecy")
The D7 class was almost predatory in its appearance, with a spread-wing primary hull, long neck and head-like command pod. The early Romulan variants had a large bird-of-prey design painted on the hull, identical to what was found on the Bird-of-Prey. (TOS: "The Enterprise Incident") Other Romulan D7s had red and yellow feather-like detailing painted on the upper wing and nacelle pylon. (TAS: "The Survivor", "The Practical Joker")
The bulk of the ships overall mass was incorporated in the aft section of the ship. The bridge was located on the bulbous head, which was separated from the aft section by a relatively thin neck that flared out into the wings. Jutting from below these wings were a pair of outboard warp nacelles. The ship's impulse engines were located on the caudal section of the ship. (TOS: "Elaan of Troyius").
This design had evolved from a classic design lineage dating back well into the 22nd century with the early D5-class battle cruisers. (ENT: "Judgment") In terms of dimensions, it was comparable in size to the Federation's Constitution-class starship. (TOS: "The Enterprise Incident")
During the mid-23rd century, the D7 class posed a significant threat to the Federation with the classes impressive armament consisting of two nacelle-mounted disruptor cannons, a forward-mounted phaser emitter, and a forward launcher capable of firing photon torpedoes, magnetic pulses and disruptor blasts. (VOY: "Prophecy"; TOS: "Errand of Mercy", "The Enterprise Incident"; TAS: "More Tribbles, More Troubles")
In 2269, the Klingons developed a prototype weapon, described as a projected stasis field, which was first encountered on the IKS Gr'oth by the Federation. The field could be projected in up to three different directions at one time. The weapon, which was extremely powerful, was capable of completely paralyzing an enemy vessel, including all its vital systems. It was however equally crippling to the firing ship itself. Ultimately, it was determined by the Federation that this was so impractical in standard combat, that it posed no real threat. (TAS: "More Tribbles, More Troubles")
While the D7 cruiser was often viewed as a threat to a Constitution-class starship, under certain circumstances, it was possible for a Constitution to destroy a single D7 with a barrage of phaser fire. Following such an instance at the outbreak of the Federation-Klingon War in 2267, the USS Enterprise encountered a fleet of eight D7s, which was considered by Captain James T. Kirk to have superior odds. Despite these odds, Hikaru Sulu was confident the Enterprise could have defeated them. (TOS-R: "Errand of Mercy")
Klingon ships were outfitted with cloaking devices as late as 2269. One of the first Klingon vessels to acquire the new technology was the IKS Klothos. The cloaking device on this class could be penetrated by a metaphasic sweep. (TAS: "The Time Trap"; DS9: "Once More Unto the Breach"; VOY: "Prophecy")
Some Klingon ships in the 23rd century were equipped with an S-2 graf unit, which was roughly equivalent to the warp drives in the Federation's Constitution-class starships. The warp drive of the D7 enabled it to catch up with a ship traveling at warp 9. (TAS: "The Time Trap"; TOS: "Elaan of Troyius", "The Enterprise Incident")
The conference room was used as a meeting place for the ship's senior staff for mission briefings. It contained a triangular table embossed with the emblem of the Klingon Empire, with each side of the table having its own set of chairs. Surrounding the table are various computer consoles. (TAS: "The Time Trap")
- Unnamed D7-class starships
The term "D-7" remained the speculated designation for the Klingon battle cruiser for many years, at the time simply referred to as the "Klingon Battle Cruiser" in production sources, with origins rooting to an incident Gene Roddenberry recalls in The Making of Star Trek, which is linked to how he likened the cast's relationship to that of a family:
I went to the stage one day, and they were all ready and waiting for me, because they knew I was really exhausted from some long rewrite sessions. As soon as I walked up to the set, Bill and Leonard blew a scene, but they blew it on purpose and began arguing very violently. Bill was shouting at the top of his voice, "Leonard! What do you mean saying this is a D-7 Klingon ship! It's a D-6!" Leonard shouted back, "No, you idiot, the D-6 has four doors over here and the D-7 only has two!" Bill immediately shouted back, "No, no, no – it's the other way around. You've got it all wrong."
While all of this is going on, I'm standing there, beginning to get frustrated, watching the minutes tick by and mentally counting the money we're losing in expensive crew time, because the cameras aren't rolling. And as the argument continued, I'm thinking to myself, "What are they talking about? They've gone too far!" Then I remembered thinking that I should remember which is the D-6 or the D-7. Finally I couldn't stand it any more, and so I walked in between them and said, "Come on, fellows, it really doesn't matter. Let's get on with the scene." Then the whole crew broke up laughing. This was their way of saying to me, "Hey, time is not that serious. Relax a little."
The designation was not actually referenced on screen until Star Trek's 30th anniversary, in the DS9 Season 5 episode "Trials and Tribble-ations", where Kira Nerys identified this type as a "D-7 battle cruiser", a term that also appeared on an okudagram on a number of occasions. In the VOY Season 7 episode "Prophecy", Tuvok identified this type as a "D-7 class cruiser".
The designation of D6 saw wide use in the Star Fleet Battles gaming universe and its derivatives but has not yet manifested on screen. In the gaming universe, the D6 is almost cosmetically indistinguishable from the D7 except for the addition of two more disruptor emitters and having more powerful engines. Considered dated after the introduction of D7 class, some D6s were sold to the Romulans, explaining why Romulans used Klingon vessels. D6s in the Klingon empire were relegated to light cruiser class or other variants.
- See D7-class model
- In writing the DS9 Season 7 episode, "Once More Unto the Breach", Ronald D. Moore stated that he intentionally included a nod to The Animated Series by means of a reference to the TAS Season 1 episode "The Time Trap". In this reference, the elderly Kor recalled the time when he commanded the Klothos during the Battle of Caleb IV, a vessel he described as being "one of the old D-5 cruisers." While the reference the cloaking device-equipped Klothos matched the Klothos that appeared in "The Time Trap", the design itself had actually appeared as a slight variant of the D7 class, rather than the D5 class design that was later introduced in the ENT Season 2 episode ENT: "Marauders".
- The designation of the Klingon battle cruiser that appear in the episode "Prophecy" was misidentified in the script during the ship's appearance. Writer Mike Sussman later explained the mistake in a response to an article.  He assumed that the physical model built by Jein from "Trials and Tribble-ations" could be reused in the episode. However all the effects were apparently to be computer generated. To his knowledge, a CGI version of Jein's D7 did not exist at the time and it would have cost a fair amount of money to design one. Had Sussman been aware of this, he would have simply changed the script.