The Random House Dictionary defines juxtaposition "[[SesquipedalianLoquaciousness as an act or instance of placing close together or side by side]], [[SignatureStyle esp. for comparison or contrast]]."
Many comedy writers, however, define juxtaposition as "two completely unrelated things being put together, [[ForScience just to see what could happen]]." A Juxtaposition Gag is when a character or object is placed in a normally unrelated situation or genre for the sake of a joke.
Common features of a Juxtaposition Gag are extremely obvious anachronisms (Abraham Lincoln, [[RecycledInSPACE ASTRONAUT]]) or flat-out contradictory concepts (TheGrimReaper, M.D.), but any two things that are sufficiently distinct from each other can suffice.
** Argument Clinic
** Bruce Sketch / Philosopher's Song
** The "Ministry of Silly Walks" sketch is about a government agency whose mission is to develop silly walking styles. At one point, John Cleese's character notes that the Ministry can't just dole out grants willy-nilly because the British government ''actually spends less money'' on silly walks than it does on national defense.
** Really Camp Army Parade
** The "Self Defense Against Fresh Fruit" sketch consists of a leatherlung instructor teaching his students how to defend themselves against attackers armed with fresh fruit. Defense methods include dropping 16-ton weights on the attacker and siccing large predatory animals on them.
** Taco Mail
** Slash Infestation, in which a home is plagued by the guitarist Slash. Another home at the end of the sketch is infested with [[RedHotChiliPeppers Fleas]].
** A post-apocalyptic quiz show
** Sir Digby Chicken Caesar: A tramp who thinks he is some sort of SherlockHolmes figure out to save the world.
** Some jokes on the Jay Leno "Headlines" special come from this sort of thing. Sometimes, two ads with incompatible content are placed by each other, or two ads with pictures are placed together and create improbable anatomy. Other times, the headline of one article is next to a picture from a different news story altogether, with unfortunate implications.
* TheSimpsons has the occasional ShowWithinAShow example:
** Admiral Baby, a show about a infant naval leader.
** Hail to the Chimp, a film about a chimpanzee who runs a country.