The Postseason refers to games played after the end of the regular season to determine the league's champion. From 1884 to 1900 postseason games were played only as exhibition matches. The first official World Series between the American League and the National League took place in 1903. There was no such postseason in 1904, however the tradition became permanent as of the 1905 season. The postseason's format has changed repeatedly over the years.
Up to 1968, the only postseason series was the World Series, featuring the teams that won their respective league pennants. The leagues were not yet divided into Divisions. The winners of the titles had to have the best overall record in their league. Those titles were sometimes decided by a one-game or three-game playoff, but these games were counted as part of the regular season. The World Series were generally held in a best-of-seven format, although there were exceptions in 1903 and from 1919 to 1921, when they were best-of-nine.
With the expansion of 1969, the leagues were divided into two divisions each, the Eastern and Western divisions, necessitating League Championship Series, which became the first round of the postseason. From 1969 to 1984 these series were held as best-of-five playoffs, and from 1985 until 1993 they were best-of-seven playoffs. The two winners would then meet in the World Series, which became the second round of the postseason.
There was only one exception to this scenario, and it came in the 1981 Postseason. It was caused by the 1981 strike. The strike came in the middle of the season, so a decision was made after the strike was resolved to have the leading teams of each division from before the strike - the so-called "first-half champions" - play the leaders of each division from after the strike - the "second-half champions". See 1981 Split Season Schedule.
In 1994, the leagues were further divided into three divisions: Eastern, Central, and Western. In order for postseason play to produce a winner in an even number of match-ups, a Wild Card winner was chosen from each league. This would be the team from each league that had the best overall record among those that did not finish first in their division. The three division winners along with the Wild Card winner from each league played a five-game Division Series. Ironically, the first year of this new arrangement did not go as planned. The 1994 strike cut the season short and postseason play was cancelled for the first time in 90 years. The first such postseason scenario took place in the 1995 Postseason.
In 2012 the postseason was expanded again with the creation of a second wild card team in each league. This was the scenario that prevailed until 2021. The now two wild card teams per league played each other in a one-game winner-take-all round hosted by the team with the better record - the Wild Card Game. The winner then advanced on to the Division Series to play the team with that league's best regular season record, regardless of division.
Due to the coronavirus pandemic and the shortened regular season that resulted, the postseason was modified for the 2020 season only. Under this format, eight teams qualified for the postseason from each league: the first two teams in each of the three divisions, and the two teams with the next best records. These teams were then ranked 1 through 8, with team 1 playing team 8, in the first round and so forth. The Wild Card Game was scrapped, replaced by a preliminary round played entirely in the home ballparks of teams ranked 1 to 4 in a best-of-three format, the Wild Card Series. The postseason then continued with the Division Series as usual.
Current postseason format
After reverting to the established format in 2021, Major League Baseball once again expanded the postseason starting in 2022, with 12 teams participating in each league: three division winners and three wild card teams. The postseason format is thus the following in each league:
- The top two division winners receive a bye into the Division Series round;
- Two best-of-three Wild Card Series in each league, pitting the third division winners and the three wild card teams, with the higher seed hosting all three games;
- Two best-of-five Division Series, with the two Wild Card Series winners playing the top two division winners who have received first-round byes;
- One best-of-seven League Championship Series;
- The World Series, played on a best-of-seven basis.
This format also eliminated the possibility of a one-game playoff to determine teams participating in the postseason: all ties will be resolved by a tiebreaking formula.
- Anthony Castrovince: "Underdogs? Don't tell these playoff teams: A look at the 10 biggest postseason upsets in MLB history", mlb.com, October 3, 2021. 
- Anthony Castrovince: "Complete history of baseball’s postseason formats", mlb.com, September 6, 2022. 
- Mark Feinsand: "MLB considering new format for postseason", mlb.com, February 10, 2020. 
- Jerry Lansche: Glory Fades Away: The Nineteenth-Century World Series Rediscovered, Taylor Publishing, Dallas, TX, 1991. ISBN 0-87833-726-1
- Stuart Shapiro: "Measuring Franchise Success in the Postseason", The Baseball Research Journal, SABR, Volume 44, Number 2 (Fall 2015), pp. 32-36.
- Evan Thompson: A Complete History of the Major League Baseball Playoffs - Volume I: Pre-divisional tiebreakers through 1976, BookBaby, Pennsauken Township, NJ, 2021, ISBN 9781098372804