A club's 25-man roster is its full roster of active Major League players. Typically, a club will have some combination of 12 position players and 13 pitchers or 12 pitchers and 13 position players. Players on the 25-man roster (or the Major League injured list) accrue Major League service time. All players on the 25-man roster must also be on the 40-man roster. Teams are required to have a minimum of 24 players on their 25-man roster.
In the case of doubleheaders, clubs are allowed to recall one additional player who is on the 40-man roster to serve as the 26th member of the active Major League roster only for the day of said doubleheader. That player accrues one day of MLB service time and is returned to the Minor Leagues following the completion of the doubleheader.
Upon being optioned to the Minor Leagues, a player must remain there for a minimum of 10 days before he is eligible to be recalled to the Major League roster, unless he is serving as the 26th man for a doubleheader or replacing a player who has been placed on the injured list. In these exceptions, there are no minimum number of days in which the optioned player must remain in the Minors.
On Sept. 1, teams can carry up to 40 players on their active roster until the start of postseason play, at which point the maximum reverts back to 25. All players added to the active roster during this period must be on the 40-man roster.
Impending rule changes that will go into effect at the start of the 2020 season
Beginning in 2020, the roster size from Opening Day through Aug. 31 (and during the postseason) will increase from 25 to 26, with the minimum number of players on the roster increasing from 24 to 25, and roster sizes for doubleheaders increasing from 26 to 27. The minimum number of days a player will need to remain in the Minors after being optioned will increase from 10 to 15, with the same exceptions in play.
The 40-man active roster for September will be eliminated. From Sept. 1 through the end of the regular season, all clubs must carry 28 players.
Furthermore, the number of pitchers a club can carry on the active roster will be capped (maximum number TBD). To adhere to that rule, clubs will have to designate each player as either a pitcher or a position player prior to each player's first day on the active roster for a given season. That designation cannot change for the remainder of the season.
Position players will not be allowed to pitch except in the following scenarios:
• They are designated as a "Two-Way Player." A player can only qualify for this designation if he accrues at least 20 Major League innings pitched and at least 20 Major League games started as a position player or designated hitter (with at least three plate appearances in each of those games) in the current or prior season.
• A game goes into extra innings.
• A player's team is losing or winning by more than six runs when he enters as a pitcher.