Major League Baseball - League Organization

League Organization

As of the 2013 season, Major League Baseball is divided into the American League (15 teams) and the National League (15 teams).

Currently, each league is further subdivided into three divisions — labeled East, Central, and West. The three-division structure dates back to 1994, the first season after the National League expanded to 14 teams. From 1969 through 1993, each league consisted of an East and West division. Through 1996, the two leagues met on the field only during the World Series and the All-Star Game: in 1997, regular-season interleague play was introduced.

In March 1995, two new franchises — the Arizona Diamondbacks and Tampa Bay Devil Rays — were awarded by Major League Baseball, to begin play in 1998. This addition would bring the total number of franchises to 30. In early 1997, Major League Baseball decided to assign one new team to each league: Tampa Bay joined the American League and Arizona joined the National League. The original plan was to have an odd number of teams in each league (15 per league, with 5 in each division). MLB also planned to introduce interleague play in 1997, but — with each league having an odd number of teams — interleague play would have had to be used throughout the entire season, to allow every team to play every day. It was unclear, though, if interleague play would continue after the 1998 season, as it had to be approved by the players' union. For this and other reasons, it was decided that both leagues should continue to have an even number of teams; one existing club would have to switch leagues. The Milwaukee Brewers agreed in November 1997 to move from the American League to the National League, thereby making the National League a 16-team league.

Following the 2011 season, Major League Baseball announced its plan to move the Houston Astros from the NL Central to the AL West for the 2013 season, resulting in both leagues having three divisions of five teams each and allowing all teams to have a more balanced schedule. (MLB required the Astros to accept this move as a condition of approving their sale to Jim Crane.) Because each league will have an odd number of teams, interleague play will occur throughout the season, so that every team will be able to play every day.

The two leagues were once separate, rival corporate entities, but that distinction has all but disappeared. In 1903, the two leagues began to meet in an end-of-year championship series called the World Series. In 1920, the weak National Commission, which had been created to manage relationships between the two leagues, was replaced with the much more powerful Commissioner of Baseball, who had the power to make decisions for all of professional baseball unilaterally. In 2000, the American and National Leagues were dissolved as legal entities, and Major League Baseball became a single, overall "league" de jure (albeit with two components called "leagues"), although it had operated as a de facto single entity for many years.

The same rules and regulations are used in both leagues, with one exception: the American League operates under the Designated Hitter Rule, while the National League does not. This difference in rules between leagues is unique to MLB; the other sports leagues of the US and Canada have one set of rules for all teams.