This separation gradually caused the leagues to develop slightly different strategies and styles of play.

The NL's adoption of the designated hitter rule in 2022, and the expansion of interleague play to 46 games beginning in 2023, has further blurred the lines between leagues.

Though both leagues agreed to be jointly governed by a commissioner in 1920, they remained separate legal and business entities with their own president and management.

By 1875, the National Association of Professional Base Ball Players (NAPBBP, often referred to as the "National Association"), founded four years earlier, was suffering from a lack of strong authority over clubs, unsupervised scheduling, unstable membership of cities, dominance by one team (the Boston Red Stockings), and an extremely low entry fee ($10) that gave clubs no incentive to abide by league rules when it was inconvenient to them.

Hulbert had a great vested interest in creating his own league, and after recruiting St. Louis privately, four western clubs met in Louisville, Kentucky, in January 1876.

The Athletic and Mutual clubs fell behind in the standings and refused to make western road trips late in the season, preferring to play games against local non-league competition to recoup some of their financial losses rather than travel extensively incurring more costs.

Hulbert reacted to the clubs' defiance by expelling them, an act which not only shocked baseball followers and the sports world (since New York and Philadelphia were the two most populous cities in the league), but made it clear to clubs that league scheduling commitments, a cornerstone of competitive integrity, were not to be ignored.

When all eight participants for 1881 returned for 1882—the first off-season without turnover in membership—the "circuit" consisted of a zig-zag line connecting the eight cities: Chicago, Detroit, Cleveland, Buffalo, Troy (near the state capital of Albany, New York), Worcester (Massachusetts), Boston, and Providence.

Both teams remain in the NL today, the Phillies in their original city and the New York franchise (later named Giants) now in San Francisco since 1958.

The NL struck back by establishing new clubs in 1883 in AA cities Philadelphia (later called "Phillies") and New York (the team that would become the Giants).

The National League and the American Association participated in a version of the World Series seven times during their ten-year coexistence.

Starting with the Pittsburgh Pirates in 1887, the National League began to raid the American Association for franchises to replace NL teams that folded.

The Union Association was established in 1884 and folded after playing only one season, its league champion St. Louis Maroons joining the NL.

The Brooklyn, Chicago, Pittsburgh, and New York franchises of the NL absorbed their Players' League counterparts.

After the 1891 season, the AA disbanded and merged with the NL, which became known legally for the next decade as the "National League and American Association".

The teams now known as the Cincinnati Reds, Los Angeles Dodgers (originally Brooklyn) and Pittsburgh Pirates (as well as the now-defunct Cleveland Spiders) had already switched from the AA to the NL prior to 1892.

The other two pre-1892 teams still in the league are the Philadelphia Phillies and the San Francisco Giants (originally New York), both of which joined in 1883.

The league became embroiled in numerous internal conflicts, not the least of which was a plan supported by some owners (and bitterly opposed by others) to form a "trust", wherein there would be one common ownership of all twelve teams.

In addition to fighting each other, they fought with the umpires and often filled the air at games with foul language and obscenities.

Billy Sunday, a prominent outfielder in the 1880s, became so disgusted with the behavior of teammates that he quit playing in 1891 to become one of America's most famous evangelical Christian preachers.

This meant formal acceptance of each league by the other as an equal partner in major-league baseball, mutual respect of player contracts, and an agreement to play a postseason championship—the World Series.

Four days later, on November 12, both sides met (without Johnson) and agreed to restore the two leagues and replace the ineffective National Commission with a one-man Commissioner in the person of federal Judge Kenesaw Mountain Landis.

In 1958 the Brooklyn Dodgers and the New York Giants moved to Los Angeles and San Francisco, respectively, bringing major league baseball to the West Coast of the U.S. for the first time.

[7] As the collective bargaining agreement came closer to expiring after the 2021 season, owners expressed their intentions to use the designated hitter in all games starting in 2022.

[8] In response to the COVID-19 pandemic in 2020, rule changes were instituted in both leagues for the 2020 season, which included an abbreviated 60-game schedule, the use of the designated hitter in all games, and expanded rosters.

As a condition of the sale of the Astros to Jim Crane in November 2011, the team agreed to move to the American League effective with the 2013 season.

Due to the 1994–95 Major League Baseball strike, there were no pennant or World Series winners in 1994, so this year is left blank.