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 1903 (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 1 in Canada (the Toronto Blue Jays). 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, if a tie breaker is 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. MLB is controlled by the Major League Baseball Constitution that has undergone several incarnations since 1876 with the most recent revisions being made in 2005. Under the direction of Commissioner of Baseball (currently Rob Manfred), Major League Baseball hires and maintains the sport's umpiring crews, and negotiates marketing, labor, and television contracts. As is the case for most of the sports leagues in the United States and Canada, the "closed shop" aspect of MLB effectively prevents the yearly promotion and relegation of teams into and out of Major League Baseball by virtue of their performance. Major League Baseball maintains a unique, controlling relationship over the sport, including most aspects of minor league baseball. This is due in large part to a 1922 U.S. Supreme Court ruling in Federal Baseball Club v. National League, which held that baseball is not interstate commerce and therefore not subject to federal antitrust law. This ruling has been weakened only slightly in subsequent years.