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In baseball, an ace is the best starting pitcher on a team and nearly always the first pitcher in the team's starting rotation. Barring injury or exceptional circumstances, an ace typically starts on Opening Day. In addition, aces are usually preferred to start crucial playoff games, sometimes on three days rest.
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".
In the early days of baseball, the term "ace" was used to refer to a run.
A lot of 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.