Major League Baseball (MLB) is a professional baseball organization that is one of the major professional sports leagues in the United States and Canada. It comprises 30 teams, divided equally between the National League (NL) and the American League (AL), with 29 in the United States and 1 in Canada. The NL and AL acted as independent organizations from their founding in the 19th century, but they merged in 1903 to form MLB, making it the oldest major professional sports league in the world. A baseball game is played between two teams, each usually composed of nine players, that take turns playing offense (batting and baserunning) and defense (pitching and fielding). A game consists of nine innings, and the team with the most runs at the end of the game wins. The MLB champion is determined by playoffs that culminate in the World Series. Baseball is often referred to as America's National Pastime and has had a very active role in the shaping of the United States. It is closely tied to American history and is an American family tradition.