Major League Baseball is a professional baseball organization, the oldest of the four major professional sports leagues in the United States and Canada. A total of 30 teams play in the National League and American League, with 15 teams in each league. The NL and AL were formed as separate legal entities in 1876 and 1901 respectively. After cooperating but remaining legally separate entities beginning in 1903, the leagues merged into a single organization led by the Commissioner of Baseball in 2000. The organization also oversees Minor League Baseball, which comprises 256 teams affiliated with the Major League clubs. With the World Baseball Softball Confederation, MLB manages the international World Baseball Classic tournament. Baseball's first openly all-professional team was founded in Cincinnati in 1869. The first few decades of professional baseball were characterized by rivalries between leagues and by players who often jumped from one team or league to another. The period before 1920 in baseball was known as the dead-ball era; players rarely hit home runs during this time.