A couch gag is a running visual joke in the Opening Sequence of The Simpsons. The couch gag changes from episode to episode, sometimes with the placement of the couch changing. In more recent seasons, the couch gags have tended to be more outlandish and absurd, sometimes showing the Simpson family sitting on the couch in different types of media (claymation, live-action, flipbook animation, etc.).
Generally, between one-half and two-thirds of the couch gags used in a season are new, while the remaining couch gags are repeats. Most couch gags are used at least twice, with a second occurrence usually in the same season as the first. However, as of Season 21, couch gags are rarely reused.
The first episode of the series to air, "Simpsons Roasting on an Open Fire" (episode 7G08), didn't feature a couch gag. The second episode (aired and produced), "Bart the Genius", features the first couch gag to be aired. When the family sits down on the couch, Bart is squeezed off the couch and pops up into the air. During the shot of the television set following the couch shot Bart is seen falling back down in front of the TV. This was the first and remains one of the few couch gags that continues into the closing shot of the TV.
"Some Enchanted Evening" featured a couch gag where the family sits normally, with nothing else happening. "Some Enchanted Evening" was to be the first episode of the series, meaning if it was the first episode to air, its couch gag would've been the first instead of the one in "Bart the Genius".
Extra long couch gags
The most-used couch gag in first-run primetime airings, the "Circus Line couch gag" (first used in "Lisa's First Word"), involves the family forming a chorus line with a group of dancers. The living room background makes way to reveal unicyclists and elephants in a large production number, while a circus-themed version of the theme song is heard. Since it was used in a total of eight episodes to date, this extra-long couch gag has mostly been used when episodes were in danger of being too short. The extra length of the gag would compensate for the shorter episode. This has become less common in more recent seasons, as the required length for each episode has been reduced due to additional advertising.
Another extra-long couch gag is when the family tries to sit on the couch, but it tries to attack them. Other couches attack people (though Moe is able to fight them off by shooting them with his shotgun), with the closing scene has Homer going into a couch store, sitting down, and being attacked. This couch gag was first seen on "Marge's Son Poisoning", then reran on the Season 17 finale "Marge and Homer Turn a Couple Play".
"Beware My Cheating Bart" has what was the longest couch gag on the show, beating out the previous record held by Season 18's "Homerazzi"'s evolution gag by eight seconds (the current longest couch gag is the Rick and Morty Couch Gag from Season 26's "Mathlete's Feat"). The couch gag features Homer falling in love with the couch, until he meets Marge. He ends up neglecting the couch, and it ends up resorting to stripping and prostitution in order to support itself and its baby couch. The couch attempts suicide by jumping into the back of a garbage truck, but Homer ends up removing it and bringing it back to his house. The baby couch is now Maggie's chair. The couch gag was animated by Bill Plympton, becoming the third couch gag to be directed by a guest after graffiti artist Banksy directed the couch gag for "MoneyBART" and John Kricfalusi directed the couch gag for "Bart Stops to Smell the Roosevelts".
The first couch gag to be aired in HD in "Take My Life, Please", the "Simpsons Chasing Couch couch gag", is also an extra-long couch gag. The Simpson family notices the couch is missing and they chase it through lots of places, including the space, where they finally catch the couch and send it back to their house where Homer turns the TV on.
In the 500th episode ("At Long Last Leave"), it shows the couch gags from "Bart the Genius", "Homer's Odyssey", "Moaning Lisa" and "Bart Gets an "F"" in normal speed (with the first two full and the latter two shortened), followed by a very quick montage of the other couch gags in all of the previous Simpsons episodes from the couch gag from episode #15 "Simpson and Delilah" to episode #499 "The Daughter Also Rises". After that, the family sits on the couch as usual, but the camera pans out to reveal that all the couch gags (including the shot of the family from earlier) are in a mosaic that has "500" on it. The mosaic then breaks to reveal Homer strangling Bart.
Other couch gags
One of the most memorable couch gags involved the Simpsons arriving to find The Flintstones sitting on the couch. It was first featured in Season 4's "Kamp Krusty", but was later used in repeat airings (and in international airings) of "The Itchy & Scratchy & Poochie Show" (which originally used the couch gag from "Bart After Dark" where the living room is a detailed replica of the Beatles "Sgt. Pepper" album, featuring every character and object from the show and statues of the Simpsons as they were on the Tracy Ullman Show cartoon shorts) to commemorate the show beating the The Flintstones' 166-episode mark as America's longest-running animated primetime sitcom.
A reference to another show by Matt Groening, Futurama, was made in the couch gag for Season 12's "HOMЯ" where the family is deposited on the couch via the travel tubes that appear in New New York City, but Philip J. Fry comes down the chute instead of Bart (which is quickly corrected when Fry gets sucked up and Bart comes down).
For the Season 19 premiere, "He Loves to Fly and He D'ohs", the entire opening sequence was modified as a callback to The Simpsons Movie. The couch gag features the family running in to see Plopper sitting on the couch. Homer lovingly picks up the pig and says, "My summer love", followed by Plopper squealing.
The most frequently used couch gag in syndicated versions of Simpsons episodes from seasons one to five is the Duplicate Family couch gag.