Major League Baseball rivalries

In the "Original 16" era (1901–60), there were eight teams in each league and teams in each league played each other 22 times a season. With the second American League incarnation of the Washington Senators (now the Texas Rangers) and Los Angeles Angels (now the Los Angeles Angels of Anaheim) entering play as expansion teams in 1961, MLB increased the total number of games American League teams played to 162, which meant teams would play each other 18 times a season. The National League did not implement this until the following year when the New York Mets and Houston Colt .45s (now the Houston Astros) entered play.

In 1969, with the San Diego Padres, Seattle Pilots, Kansas City Royals, and Montreal Expos entering play as expansion teams, MLB split both leagues into two divisions with six teams each. Teams played a total of 90 intra-divisional games, playing teams within the division 18 times each and 72 inter-divisional games, playing each team in the other division 12 times. However, in 1977, the addition of the Seattle Mariners and Toronto Blue Jays reduced the number of intra-divisional games American League teams played to 78, as each team would play each team within the division 13 times. However, they still played each team in the other division 12 times, but the total number of inter-divisional games increased to 84. The National League did not institute this until 1993, when the Florida Marlins and Colorado Rockies entered play.

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