Expansion Era (since 1961)

Definition

As the demographics of the United States changed throughout the first half of the 20th century, particularly after World War II, Major League Baseball followed suit, expanding beyond the original 16 teams of the American and National Leagues in 1961.

That’s when four new clubs were established -- the Houston Colt .45s (now the Astros), the New York Mets (a team that took the place of the Dodgers and Giants after those franchises moved from New York to California in 1958), the Washington Senators (to replace the former Washington club that relocated to Minnesota and became the Twins) and the Los Angeles Angels.

The Senators and Angels began play in the AL in 1961, and the Astros and Mets began play in the NL in 1962. In 1972, the Senators moved to Texas and became the Rangers.

Later expansion

MLB expanded by four teams again in 1969, including the first Major League team in Canada. The Montreal Expos and San Diego Padres each began play in the NL in 1969, while the Kansas City Royals and Seattle Pilots (who became the Milwaukee Brewers after one season) began play in the AL the same year.

In 1977, the AL debuted two new clubs, the Seattle Mariners and the Toronto Blue Jays. That brought the total number of Major League franchises to 26 -- 14 in the AL and 12 in the NL.

In 1991, Denver and Miami were each awarded Major League clubs. The Colorado Rockies and the Florida Marlins (now Miami Marlins) began play in the NL in 1993, balancing the number of teams in each league at 14 apiece.

In 1998, the Arizona Diamondbacks and the Tampa Bay Devil Rays (now the Tampa Bay Rays) each played their inaugural seasons, bringing the total number of MLB franchises to the current 30 -- Arizona’s franchise brought the total number of NL clubs to 15, and Tampa Bay’s franchise brought the AL total to 15.

But because an odd number of clubs in each league would require Interleague games daily, the Brewers moved to the NL so that the NL would have 16 clubs and the AL would have 14.

In 2013, the Astros moved to the AL, once again equalizing the number of clubs in each league and necessitating at least one Interleague game each day of the season on which there was a full slate of games scheduled.