Each League is divided into the East, Central and West divisions and each division is composed of 4 – 6 teams. There are 15 teams in the National League and 15 teams in the American League. While American League has a designated hitter system, the National League does not.
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. For 60 years, the American and National Leagues fielded eight teams apiece.
In the 1960s, MLB expansion added eight teams, including the first non-U.S. team (the Montreal Expos). Two teams (the Seattle Mariners and the Toronto Blue Jays) were also added in the 1970s. From 1969 through 1993, each league consisted of an East and West Division. A third division, the Central Division, was added in each league in 1994. Through 1996, the two leagues met on the field only during the World Series and the All-Star Game. Regular-season interleague play was introduced in 1997.
In March 1995, two new franchises—the Arizona Diamondbacks and Tampa Bay Devil Rays (now known simply as the Tampa Bay Rays)—were awarded by MLB, to begin play in 1998. This addition brought the total number of franchises to 30. In early 1997, MLB decided to assign one new team to each league: Tampa Bay joined the AL and Arizona joined the NL. The original plan was to have an odd number of teams in each league (15 per league, with 5 in each division). In order for every team to be able to play daily, this would have required interleague play to be scheduled throughout the entire season. However, it was unclear at the time 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 AL to the NL, thereby making the NL a 16-team league. Later, when the Houston Astros changed ownership prior to the 2013 season, the team moved from the NL Central to the AL West, resulting in both leagues having three divisions of five teams each and allowing all teams to have a more balanced schedule. Interleague play is now held throughout the season.