On both the Galaxy- and Sovereign-classes, the room was elongated, spanned the entire area aft of the bridge on Deck 1 and featured a large conference table. There was also a series of seven viewports, composed of transparent aluminum. (TNG: "In Theory") These viewports spanned from close to the floor to up along the angled ceiling, which faced the rear of the ship. (TNG: "Lonely Among Us")
The observation lounge on the Galaxy-class was connected to the bridge through a short, ramped corridor, which originated at the upper starboard alcove of the bridge and led to a door on the starboard side of the conference lounge. Another door, on the port side of the room, led to another corridor, (TNG: "Datalore") at the end of which was a turbolift. The shape of the conference table matched the curvature of the exterior hull, and had seating and data pads for ten individuals. The captain usually sat at the head of the table, at either side. In 2365, two display screens were added and located at either side of the room, behind the table and adjacent to the doors. The observation lounge was decorated by plants, and had two replicator terminals. (TNG: "The Child")
The Enterprise-D's observation lounge was scripted to first be shown on-screen during the initial scenes of "Encounter at Farpoint", while Picard tours the ship. However, the room does not appear during this sequence. In fact, the only scene of "Encounter at Farpoint" that is set in the location was not scripted to be, as that scene was intended to be in Picard's cabin instead. 
Different angles of the conference lounge were shown on TNG and, when directing episodes of the series, Les Landau was careful to make varied use of the room. "I remember he was always looking for new ways to shoot the observation lounge," offered Michael Okuda. ("Tapestry" audio commentary, TNG Season 6 Blu-ray) However, filming in the set was notoriously difficult. (Cinefantastique, Vol. 25/26, No. 6/1, p. 83)
In 2366 of an alternate timeline, the battleshipEnterprise-D had an observation lounge which omitted the display of Enterprise models and seemed to have only one door. However, the conference table was still a feature of the room, as was a row of windows built into one of the walls. One addition was a large tactical graphic of the Enterprise-D, illustrated in blue and yellow on a transparent, vertical stand. (TNG: "Yesterday's Enterprise")
The script for "Yesterday's Enterprise" specifies that this observation lounge "has been converted into a war room atmosphere." However, the script also describes the conference table as "filled with wartime displays and charts," which is not true of the table in the episode's final version.  Of the room, Ronald D. Moore remarked, "It's unrecognizable, with the angles and whatever they've done production-wise. You don't see the big wall with the gold ships on it, any more. You're not even really clear what set you're in, it looks so different from the standard obs lounge." Regarding the Enterprise-D tactical graphic, Michael Okuda explained, "I think it was made for ['Yesterday's Enterprise']." The graphic was reused in the teaser of "Captain's Holiday". ("Yesterday's Enterprise" audio commentary, TNG Season 3 Blu-ray)
The display of Enterprises was removed, for unspecified reasons, in 2368. Also at this time, ribbed light vents were added below the windows. (TNG: "Redemption II", "Darmok")
In 2369, the display screens were permanently illuminated, with the port display showing information about the Enterprise systems and the starboard display giving status reports of various Starfleet vessels. (TNG: "Chain of Command, Part I")
Curiously, no one thought to switch off the displays containing this vital information while Cardassian officers were kept in the room during "Chain of Command, Part I". Also, the modifications made to the observation lounge were retained for all past scenes included in TNG: "All Good Things...", although the sculptures of the ships were returned to the back wall. "In fact," recalled Ronald D. Moore, "I loaned them back to the company to do that [....] They had to ask my permission to bring them back in for that show." (All Good Things (Blu-ray)audio commentary)
In late 2367, the observation lounge was decompressed after the transparent aluminum of a window was temporarily phased out of normal space by pockets of subspace instability in a dark matter nebula. (TNG: "In Theory")
During the first season of TNG, the conference lounge was a redress of the sickbay set; in sickbay scenes, the outlines of the windows can be distinguished over the biobeds. Set Decorator John Dwyer recalled how the set's transformation into its observation lounge variant was achieved; "We hauled most of the sickbay stuff up into the ceiling, took out the half-round wall on one side and put in a half-round window wall, and then we brought in our banana table and all the stuff."
The scenes that take place in the observation lounge were a subject of ridicule from the TNG writing staff, the members of which occasionally visited the set and discussed stories there. "We used to make fun of what we call 'the obs lounge scenes,' because those are generally scenes," said Brannon Braga, with a laugh, "where a lot of exposition came out and, in more unfortunate episodes, technobabble." Ronald D. Moore elaborated, "The obs lounge scenes tended to be written almost by a patter, like you could really almost just write it rhythmically. Picard sitting down. 'Report.' And then, Riker gives the report and then Geordi says, 'Well, the TECH problem with this is that it's gonna blow up the ship for TECH reasons,' and then Data says, 'But there is a theory, of how we could TECH the TECH.' And then Riker says, 'Has it ever been done before?' And Geordi says, 'No, but he's right; if we just TECH the TECH,' and then Picard eventually says, 'Make it so.'" (All Good Things (Blu-ray)audio commentary)
While directing "Genesis", Gates McFadden was hampered by filming in the observation lounge set. "What everyone said was true, the observation lounge was the hardest [set] to shoot," she admitted. (Cinefantastique, Vol. 25/26, No. 6/1, p. 83)
The layout of the Sovereign-class observation lounge was similar to that of the Galaxy-class; the conference table was curved in a similar manner, and there was also the same number of seats. However, the room connected directly to the bridge via two doors on either side of the master systems display. Steps inside the lounge led down to the table level. The room also lacked viewscreens during the early 2370s.
In 2373, the USS Enterprise-E conference lounge also had a "past Enterprises" display, though it took the form of a large glass case enclosing seven full models on stands (the Enterprise-D being the most recent). The display was damaged when Captain Jean-Luc Picard smashed the case with a phaser rifle in a fit of hysteria following an attack from his one-time abductors, the Borg. (Star Trek: First Contact)
By 2379, the case was split to either side of a new, large computer terminal and the doors were relocated. (Star Trek Nemesis)
The Enterprise-E observation lounge was built on Paramount Stage 29 and was an extensive redress of the TNG observation lounge, having been spared from destruction in Star Trek Generations. That revamped set was later connected to the bridge for the first time in Star Trek: First Contact. For its appearance in the ENT finale, it was restored to its TNG appearance.
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