September call-up

A Major League Baseball roster is a roster of players allowed by league agreement to play for their respective Major League team. There are two types of rosters in general, the "active roster" of 25 players and the "expanded roster" of 40 players.

Since 1910, when teams were first allowed to carry players under contract in excess of those allowed to participate in regular season games, the latter has been called the "active roster." With exceptions through the years for varying economic conditions (primarily during World War I, the Great Depression, post-World War II and from 1986-1989 when the limit was made 24 because of rising player salaries), the active roster has allowed up to 25 players to participate for a Major League team within specified dates, currently Opening Day to September 1. In 1968 the 25-player maximum for active rosters was made a part of the first collective bargaining agreement between the major leagues and the Major League Baseball Players Association. Since 1977 teams have been required to carry a minimum of 24 players on their active rosters as well.

Active rosters include the starting eight (AL) or nine (NL) position players, pitchers, and reserve players on the team. Players on the active roster are also on the 40-man roster. These players are generally the only ones who dress in uniform and are the only ones who may take the field in a game at any time.

Typically, in modern-day play, an active roster will consist of five starting pitchers, seven relief pitchers, two catchers, six infielders, and five outfielders. Teams can vary this somewhat according to preference and circumstance, and indeed the "typical" roster makeup has changed somewhat over the years. (Starting rotations used to consist of four pitchers, not five, well into the 1970s; third-string catchers used to be much more common; many other minor variations exist.) In the American League, a full-time designated hitter is usually classified as either an infielder or an outfielder, not a DH, because most DHs do play defensive positions from time to time.

Since the 2012 season, teams have been allowed 26-player active rosters for "day-night" doubleheaders—two games scheduled on the same day, but with the stadium cleared between games, and separate tickets sold for each game—as long as the doubleheaders have been scheduled with at least 48 hours notice.

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Wikipedia