A club's 26-man roster is its full roster of active Major League players from Opening Day through Aug. 31, and during the postseason.
Teams are limited to carrying 13 pitchers during this time. From Sept. 1 through the end of the regular season, all clubs must carry 28 players, with a limit of 14 pitchers.
Players on the 26-man roster (or the Major League injured list) accrue Major League service time. All players on the 26-man roster must also be on the 40-man roster. Teams are required to have a minimum of 25 players on their 26-man roster.
MLB teams must designate every player on the active roster either as a pitcher or a position player. Those designated as position players are unable to pitch unless it is extra innings or their team is ahead or trailing by more than six runs when they take the mound. Teams can also designate players as two-way players if they meet certain criteria. Two-way players are able to pitch in any situation but don't count toward the active roster's pitcher total.
In the case of doubleheaders, clubs are allowed to recall one additional player who is on the 40-man roster to serve as the 27th 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. Teams also are permitted to call up a 27th man on days when they are completing a previously suspended game and have another game scheduled after that, but the 27th man is only permitted to play in the regularly scheduled game that day, not the contest that was previously suspended.
Upon being optioned to the Minor Leagues, a position player must remain there for a minimum of 10 days before he is eligible to be recalled to the Major League roster. For pitchers and two-way players, the minimum is 15 days. If a player is serving as the 27th man for a doubleheader or replacing a player who has been placed on the injured list, there is no minimum number of days for which the optioned player must remain in the Minors.