ALCS

The American League Championship Series (ALCS) is a best-of-seven series played in October in the Major League Baseball postseason that determines the winner of the American League (AL) pennant. The winner of the series advances to play the winner of the National League (NL) Championship Series (NLCS) in the World Series, Major League Baseball's championship series.

It started in 1969, when the AL reorganized into two divisions, East and West. The winners of each division played each other in a best-of-five series to determine who would advance to the World Series. In 1985, the format changed to best-of-seven. In 1994, the league was restructured into three divisions, with the three division winners and a wild-card team advancing to a best-of-five postseason round, known as the American League Division Series (ALDS). The winners of that round then advanced to the best-of-seven ALCS. In 2012, the playoffs were expanded again so that two wild card teams face off in a one-game wild card round to determine which team advances to the division series, with the playoffs then continuing as it had before 2012 (though with the possibility of a fifth seed being in the playoffs and a fourth seed being out) after the end of the wild card round. This is the system currently in use.

The ALCS and NLCS, since the expansion to best-of-seven, are always played in a 2–3–2 format: Games 1, 2, 6, and 7 are played in the stadium of the team that has home field advantage, and Games 3, 4, and 5 are played in the stadium of the team that does not. The series concludes when one team records its fourth win. Since 1998, home field advantage has been given to the team that has the better regular season record, unless that team happens to be the Wild Card team. In that case, the other team gets home field advantage, because by rule the Wild Card team is never allowed home field advantage in a Division Series or LCS. In the event that both teams have identical records in the regular season, home field advantage goes to the team that has the winning head-to-head record. From 1969 to 1993, home field advantage alternated between the two divisions, and from 1995 to 1997 home field advantage was determined before the season.

...
Wikipedia