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The Division Series was implemented in 1981 as a result of a midseason strike, with the first place teams before the strike taking on the teams in first place after the strike. After 1993, it was implemented for good when Major League Baseball restructured each league into three divisions, but their next playing was in 1995 due to the cancellation of the 1994 playoffs. In 1981, a split-season format forced the first ever divisional playoff series, in which the New York Yankees won the Eastern Division series over the Milwaukee Brewers (who were in the American League until 1998) in five games while in the Western Division, the Oakland Athletics swept the Kansas City Royals (the only team with an overall losing record to ever make the postseason). The Yankees have currently played in the most division series in history, with eighteen appearances.
From 1998 to 2011, the wild-card team was assigned to play in the division winner with the best winning percentage (outside of their own division) in one series, and the other two division winners met in the other series. However, if the wild-card team and the division winner with the best record were from the same division, the wild-card team played the division winner with the second-best record, and the remaining two division leaders played each other. Beginning with the 2012 season, the wild card team that advances to the Division Series was to face the number 1 seed, regardless of whether or not they are in the same division. The two series winners move on to the best-of-seven ALCS. Home field advantage goes to the team with the better regular season record, except for the wild card team, which never receives the home field advantage.
Beginning in 2007, MLB has implemented a new rule to give the team from the league that wins the All-Star Game with the best regular season record a slightly greater advantage. In order to spread out the Division Series games for broadcast purposes, the two ALDS series follow one of two off-day schedules. Starting in 2007, after consulting the MLBPA, MLB has decided to allow the team with the best record in the league that wins the All-Star Game to choose whether to use the seven-day schedule (1-2-off-3-4-off-5) or the eight-day schedule (1-off-2-off-3-4-off-5). The team only gets to choose the schedule; the opponent is still determined by win-loss records.
Initially, the best-of-5 series played in a 2-3 format, with the first two games set at home for the lower seed team and the last three for the higher seed. Since 1998, the series has followed a 2-2-1 format,  where the higher seed team plays at home in Games 1 and 2, the lower seed plays at home in Game 3 and Game 4 (if necessary), and if a Game 5 is needed, the teams return to the higher seed's field. When MLB added a second wild card team in 2012, the Division Series re-adopted the 2-3 format due to scheduling conflicts. It reverted to the 2-2-1 format in 2013.