The Major League Baseball relocations of the 1950s–1960s brought several Major League Baseball franchises to the Western and Southern United States, expanding the league's geographical reach. This was in stark contrast to the early years of modern baseball, when the American League placed teams in National League cities. Chicago, Boston, Philadelphia, and St. Louis had two teams; New York City had three. With no teams west of St. Louis or south of Washington, D.C., baseball was effectively confined to the Northeast and Midwest.

Major League Baseball (MLB) is a professional baseball league and the highest level of organized baseball in the United States and Canada. One of the big four major leagues, MLB comprises 30 teams, divided equally between the National League (NL) and the American League (AL), with 29 in the United States and 1 in Canada. Formed in 1876 and 1901, respectively, the NL and AL cemented their cooperation with the National Agreement in 1903, making MLB the oldest major professional sports league in the world. They remained legally separate entities until 2000, when they merged into a single organization led by the Commissioner of Baseball. MLB is headquartered in Midtown Manhattan.