In baseball, an ace is a starting pitcher considered the best pitcher on a team's starting rotation. Barring injury or exceptional circumstances, a team's ace typically starts on Opening Day. In addition, aces are usually preferred to start crucial playoff games, sometimes on three days' rest.^[1]

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The term may be a derivation of the nickname of Asa Brainard (real first name: "Asahel"), a 19th-century star pitcher, who was sometimes referred to as "Ace".^[2][3]

In the early days of baseball, the term ace was used to refer to a run.^[4]

Modern baseball analysts and fans have started using the term ace to refer to the elite pitchers in the game, not necessarily to the best starting pitcher on each team.^[5] For example, the April 27, 1981, Sports Illustrated cover was captioned "The Amazing A's and Their Five Aces" to describe the starting rotation of the 1981 Oakland Athletics.^[6]