For the type of architectural structure, please see archway.

An arch was a structure in 24th century Starfleet holodecks which surrounded their entrance doors. It had computer control panels and interfaces when participants wished to modify the current program, communicate throughout the ship, or access most general computer functions. (Star Trek Generations; TNG: "Elementary, Dear Data")

When a holographic program was running, the arch was typically hidden holographically. It could be made to materialize by voice command. The arch was then visible to participants, and even though the holographic characters generally did not perceive the arch, there were exceptions. (Star Trek Generations; TNG: "Elementary, Dear Data")

A related voice command, "Exit", was used to remove the holographic environment around the arch and holodeck door when participants wished to leave the holodeck without closing the program. (Star Trek: The Next Generation, et al.)

Although the arch appeared frequently in scenes on the holodeck on The Next Generation, it was strangely absent in most holodeck episodes on Deep Space Nine and all Voyager episodes, often requiring characters to search for manual holodeck controls somewhere in the environment during a malfunction instead. However, in "Twisted", Harry Kim uses the command "computer, arch", (which doesn't work, as there is a malfunction) but it is reasonable to assume that the Holodeck on Voyager "hides" the arch until it is required.
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