D7 class
The D7 class cruiser or D-7 battle cruiser was a 23rd century warship originally designed and used by the Klingon Imperial Fleet, before being used by the Romulan military during the late 2260s.

History 

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. ()

One of the first D7-class vessels ferried Chancellor L'Rell to meet with the  at Boreth. Several of these ships later fought the Section 31 armada in support of Discovery and . ()

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. (, et al. ) Among the mission profiles designated for this class was that of scout ship. () 

The Romulan Star Empire began using the same design by 2268. ()

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. (; ) 

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. ()

By the 2270s, the  had largely replaced the D7. () 

By 2377, the D7 had been retired "for decades." ()

Technical data

Physical arrangement 

 
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. () Other Romulan D7s had red and yellow feather-like detailing painted on the upper wing and nacelle pylon. ()

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. (). 

This design had evolved from a classic design lineage dating back well into the 22nd century with the early  battle cruisers. () In terms of dimensions, it was comparable in size to the Federation's  starship. ()

Tactical systems 
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. (; ; )

In 2269, the Klingons developed a prototype weapon, described as a projected stasis field, which was first encountered on the  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. ()

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  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. ()

By the late 24th century, these vessels were considered rather antiquated, and were identified as being inferior in firepower to a Federation  starship. ()

Cloaking device 
Like its predecessor, the Romulan Bird-of-Prey, the Romulan D7 was equipped with cloaking technology. 

Klingon ships were outfitted with cloaking devices as late as 2269. One of the first Klingon vessels to acquire the new technology was the . The cloaking device on this class could be penetrated by a metaphasic sweep. (; ; )

Propulsion systems 

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. (; )

 

In addition, the D7 impulse drive consisted of four-port engines, presumably giving the ship enhanced maneuverability at sublight speeds. ()

Shields 
D7s were equipped with standard shields, with their number three and number four shields being specifically located amidships. ()

Other systems 
Romulan D7s were outfitted with food synthesizers. ()

Interior design

Crew complement 
The average crew complement of a Klingon D7 was comparable to a Constitution class, and consisted of around 430. ()

Main bridge 
Referred to by the Romulans as "Control Central", the bridge was the nerve center of the D7 class. ()

Conference room 

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. ()

Ships commissioned 

Named
 
 
 

Unnamed
 Unnamed D7-class starships
 
 Kohlar's battle cruiser
 Kuri's battle cruiser

Appendices

Appearances 
 
 
 
 
 
 
 
 
 
 
 
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Background information

Name origins 
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 Treks 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.

Studio models 
See D7-class model

Inconsistencies 
 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 . 

 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.

External links 
 
 Klingon/Romulan Battle Cruiser at Curt Danhauser's Guide to Animated Star Trek
 

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D07 class
D07 class