Major League Baseball (MLB) is the highest level of play in North American professional baseball. Specifically, Major League Baseball refers to the organization that operates the National League and the American League, by means of a joint organizational structure that has developed gradually between them since 1901 (the National League having been in existence since 1876). In 2000, the two leagues were officially disbanded as separate legal entities with all their rights and functions consolidated in the commissioner's office. MLB effectively operates as a single league and as such it constitutes one of the major professional sports leagues of the United States. It is currently composed of 30 teams—29 in the United States and one in Canada. In conjunction with the International Baseball Federation, the MLB also manages the World Baseball Classic. Each season consists of 162 games (with an additional game, or games, in case of a tie breaker needed to determine postseason participation), which generally begins on the first Sunday in April and ends on the first Sunday in October, with the postseason played in October and sometimes into early November. The same rules and regulations are played between the two leagues with one exception: the American League operates under the Designated Hitter Rule, while the National League does not. Utilization of the DH Rule in interleague play, the All-Star and World Series games is determined by the home team's league rules. (via Wikipedia)