The World Series is the annual championship series of Major League Baseball (MLB). It has been contested since 1903 between the champion teams of the American League (AL) and the National League (NL). The winning team, determined through a best-of-seven playoff (except in 1903 and from 1919–1921, when a best-of-nine format was used), is awarded the Commissioner's Trophy.
The series is traditionally played in October, although before expansion of the regular-season schedule from 154 to 162 games the event occasionally started in late September (most recently in 1955) and the entire 1918 series took place early in that month due to the World War I "Work or Fight" order forcing an early end to that year's regular season, while some more recent editions have been contested into November due to in-season delays and expansion of earlier postseason rounds. Because the series is played in the fall (autumn) season in North America, it is often referred to as the Fall Classic.