25-man and 40-man Rosters
Each Major League Baseball team maintains both a 25-man roster and a 40-man roster of players. Players on the 25-man roster are eligible to play in official major league games throughout the season. The 40-man roster includes the players on the 25-man roster plus as many as 15 players who are either on the team's 7- or 15-day disabled list (see below), who are on paternity leave for up to 3 days, or who are in the team's minor league system. From September 1 through the end of the regular season, any player on the 40-man roster (also referred to as the "expanded roster") is eligible to play in an official regular season game. Many young players make their Major League debuts in this way, as "September call-ups."
To be eligible for a team’s playoff roster a player must be on any of the following: (a) the 25 man active roster, (b) the disabled list, (c) the bereavement list, or (d) the suspended list as of August 31 at midnight. The only exception is that a player who is on the disabled list at the end of the regular season may be replaced on the postseason roster by any other player who was in the organization on August 31, with the approval of the commissioner of baseball.
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