As of 2021, the best-of-nine format was used in how many Major League Baseball World Series championships?

A best-of-nine playoff format pits two teams head-to-head, and one must win five games to win the series. Five is chosen as it would constitute a majority of the games played. Whoever has won five games before all nine games have been played is the winner. Then all other games are omitted.

In Major League Baseball (MLB), the World Series conducted a best-of-nine playoff format in its first year of existence in 1903. Then, beginning in 1919, (the year of the "Black Sox scandal") and again in 1920 and 1921, the best-of-nine playoff format was used in the series. As of 2021, during the total history of MLB World Series championship contests, between the champions of baseball's National League (NL) and American League (AL), only four annual championship series have ever been decided by a nine game playoff series.

When the best-of-nine playoff format was used, it was reported by several national newspapers that the MLB partly picked it (as opposed to the best-of-seven) to increase popularity of the sport and/or to generate more revenue for Major League Baseball to improve overall league operations.