In organized baseball, teams are often grouped together in a league. In professional baseball at the major league level, there are two leagues. The American and National leagues represent the pinnacle of elite baseball in North America, and both of these leagues have rules by which games are played and championships are administered.
Number of Players
All teams in the American and National leagues play the majority of the season with 25 players on their roster. Teams are allowed to put 40 players on their major league roster from the time the season is over until the first day of the season. On opening day, however, the roster is trimmed to 25 players. When one player gets hurt and is placed on the disabled list, the team can bring a player up from the minor leagues and add him to the roster. When an injured player is healthy and ready to rejoin the team, another player must be removed from the roster. After games of Aug. 31 have been played, teams are allowed to play with 40 players on their roster through the end of the season. If a team is eligible for postseason play, that team can only play postseason games with 25 players.
Season Rules
Teams play 162 games during the regular season to determine division winners and a wild-card team. Each league is divided into three divisions. The teams with the best records in each of those divisions are the champions of their division. The second-place team in each league--regardless of the division--is the wild-card winner. The three division winners and the wild-card winner from each league make the postseason. The teams that play in the postseason are eligible to play in the World Series.
Postseason Format
Each league has two rounds of postseason play prior to the champion from each league meeting in the World Series. In the first round called the Divisional Playoffs, the division winner with the best regular-season record plays the wild-card winner. The other two divisional winners also meet. It is a best-of-five format. The winners of those two rounds meet in the best-of-seven League Championship Series. The winner of the American League Championship Series meets the winner of the National League Championship Series in the best-of-seven World Series.
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