The Division Series is the quarterfinal round of the Major League Baseball postseason. Four series are played in this round, two each for both the American League and the National League.
1981 season
The first use of the term "Division Series" dates from 1981, when due to a mid-season players' strike, that season was divided into two halves, with the winners of each half from each division (West and East at the time) playing one another in a best-of-five series to decide which team would represent that division in the League Championship Series (this format being common in minor-league baseball). But because the two halves of the season were independent of one another, the winner of the first half had no real incentive to try to win the second half as well (since, unlike in the minor leagues, if the same team did win both halves it was not given a bye into the next round), and a team that won neither half could have actually had the best overall record in the division; indeed, the latter actually occurred, as the Cincinnati Reds and the St. Louis Cardinals had the two best won-lost records (in both halves of the season combined) in the National League West and East respectively, with the Reds having the best overall winning percentage in all of Major League Baseball, yet neither advanced to the playoffs because they finished second in their divisions in each half. Until the 2012 format revision, this was the only Division Series which actually consisted of teams from the same division playing each other. This particular Division Series was meant as a one-off series due to the strike.
1993–1994: Proposal, realignment, and cancellation of 1994 postseason
In 1993, the owners approved the reintroduction of the Division Series, this time on a permanent basis, given the fact that three of the four series in the one-off 1981 Division Series went to a game 5. Originally, the format called for the top team in the AL East to play the 2nd place team in the AL West and vice versa. In 1994, both the National League and the American League realigned, with the number of divisions in both increasing from two to three (adding a Central Division, with fewer teams in each division). At the same time, the number of teams qualifying for baseball's postseason was doubled, from four to eight; henceforth the three first-place teams from each league's divisions would reach the postseason, along with one wild card team from each league (the latter being the second-place finisher with the best regular-season record). However, this expanded playoff format did not return until the following year, because a players' strike, which began on August 12, 1994, led to the cancellation of that season's playoffs and World Series (and caused the 1995 regular season to have 18 fewer games for each team than the standard 162 [for a total of 144 games]).
1995–1997: Pre-arranged seeding
Since its inception, the Division Series has been best-of-five; however, both the method of awarding home-field advantage in the series and which games the team getting the advantage would host were changed in 1998.
Originally, the East, Central, and West Division champions rotated home-site priority, with two division winners getting the extra home game, and third one not having the extra game along with the wild-card team, who never had it. The 2–3 format was used in which the disadvantaged team hosted the first two games, and the team with the advantage hosted the remaining game(s). This made it impossible for the disadvantaged team to clinch the series at home. A similar format had been used for the League Championship Series from 1969 to 1984. It also allowed the disadvantaged teams the unusual luxury of starting a series at home, and a guarantee that they play two games at home, whereas the advantaged team may play only one game at home if the series ends in a three-game sweep.