The D7 class Klingon battle cruiser served as the backbone of the fleet for several years during the 23rd century. (TOS: "Errand of Mercy" remastered, etc.) Among the mission profiles designated for this class was that of scout ship. (TOS: "Friday's Child", "A Private Little War" remastered)
By 2267, they were prominently featured in the Klingon Imperial Fleet, where they posed a serious threat to the security of the Federation and Starfleet. (TOS: "Elaan of Troyius"; TAS: "More Tribbles, More Troubles")
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")
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 Template:ShipClass 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"; TAS: "More Tribbles, More Troubles")
While the D7 cruiser was often viewed as a threat to a Constitution-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: "Errand of Mercy" remastered)
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")
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. (TAS: "The Time Trap"; DS9: "Once More Unto the Breach")
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 Template:ShipClass starships. The warp drive of the D7 enables it to catch up with a ship traveling at the speed of 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")
The term "D7" remained the speculated designation for the Klingon battle cruiser for many years, 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."
- 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". wbm
- 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. As a result, this usage may be interpreted as a transitional predecessor to the K't'inga in the D7 lineage.