An "opener" is a pitcher -- normally a reliever -- who starts a game for purposes of matching up against the top of the opponent's lineup in the first inning, which has traditionally been the highest-scoring inning, before being relieved by a pitcher who would otherwise function as a starter. This allows for a team to counter its opponent's first three batters with the pitcher it feels has the best chance for success against them.

The second pitcher (often called the headliner) usually pitches the bulk of the game, potentially relieved in the later innings due to high-leverage situations or high pitch count.