インターリーグ
In March 1995, two new franchises, the Arizona Diamondbacks and Tampa Bay Devil Rays (now known as the Tampa Bay Rays), were awarded by MLB, which began 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 five in each division), but 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 the 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, and therefore, one existing club would have to switch leagues. The agreed in November 1997 to move from the AL to the NL, thereby making the NL a 16-team league. At the same time, the Detroit Tigers agreed to move from the AL East to the AL Central (to replace Milwaukee), with the expansion Tampa Bay Devil Rays joining the AL East. 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.