Modern era

The Modern era distinguishes major league baseball in its modern form from the game's 19th century origins. While 19th century baseball involved much experimentation with the rules of the game, those rules had mostly evolved to their modern form by the beginning of the 20th century. One of the last significant rule changes was that pertaining to foul balls being counted as strikes with less than two strikes on a batter, with the National League making this change in 1901, and the American League in 1903.

The beginning of the modern era has come to be regarded as either the 1900 or 1901 season. The rationale for 1900 is the elimination that year of syndicate ownership in the National League. Syndicate ownership denotes the practice of one owner holding stakes in multiple franchises, an example of which was the ownership of the Cleveland Spiders and St. Louis Perfectos by Frank and Stanley Robison. The Robison brothers determined that the Perfectos were the more profitable of their two franchises and made the decision to transfer all of Cleveland's best players to the St. Louis club before the 1899 season. Unsurprisingly, the Spiders that season were no match for their competition, and won just 20 of their 154 games. Recognizing that syndicate ownership created a conflict an interest detrimental to the integrity of the game, National League syndicate owners divested themselves of their "second" franchises, resulting in the league's contraction from twelve to eight franchises for the 1900 season. Though syndicate ownership was eliminated that year, the practice continued on a lesser scale in the following decade before being formally abolished in 1910.

The alternate rationale for adopting the 1901 season as the start of the modern era is simply that this was the American League's inaugural season as a major league. While rivals to the National League had come and gone before (and after in the case of the Federal League), the American League quickly proved it had staying power, with a "modus vivendi" between the two leagues being established for the 1903 season, as well as agreement on a post-season series (that came to be known as the World Series) between the two league champions that was inaugurated the same year. Starting in 1903, the AL and NL operated the same franchises in the same cities for 50 consecutive seasons, solidly establishing modern era baseball in what would become its familiar form.