"Who's on First?" is a comedy routine made famous by American comedy duo Abbott and Costello. The premise of the sketch is that Abbott is identifying the players on a baseball team for Costello. However, the players' names can simultaneously serve as the basis for questions and responses, leading to reciprocal misunderstanding and growing frustration between the performers. Although it is commonly known as "Who's on First?", Abbott and Costello frequently referred to it simply as "Baseball".

A double act is a form of comedy originating in the British music hall tradition, and American vaudeville, in which two comedians perform together as a single act, often highlighting differences in their characters' personalities. Pairings are typically long-term, in some cases for the artists' entire careers. Double acts perform on the stage, television and film.