Before 1969, the National League champion (the”pennant winner”) was determined by the very best win-loss record in the conclusion of their regular season. There were four ad hoc three-game playoff series due to ties under this formulation (in 1946, 1951, 1959, and 1962). (The American League had to solve a tie in 1948, however, used a single-game playoff.)
A structured postseason series began in 1969, when both the National and American Leagues were reorganized into two divisions each, East and West. The two division winners in each league played in a best-of-five show to ascertain who would advance to the World Series. In 1985, the format changed to best-of-seven.
The NLCS and ALCS, because the expansion to seven games, are played at two –3–2 format: matches 1, 2, 6, and 7 have been played in the arena of the group which has home field advantage, and games 3, 4, and 5 have been played in the arena of the group that does not. Home field advantage is given to the team with the better record, with the exception that the group that produced the postseason as the Wild Card cannot get home field edge. From 1969 to 1993, home field advantage was alternated between branches each calendar year regardless of regular season record and from 1995 to 1997 home field advantage was specified prior to the season.
Back in 1981, a divisional show was held because of a split season brought on by a players’ strike.
In 1994, the team was restructured into three branches, together with the three division winners and a wild-card team advancing to a best-of-five postseason around, the National League Division Series (NLDS). The winners of the round progress to the best-of-seven NLCS.
The Milwaukee Brewers, an American League team between 1969 and 1997, along with the Houston Astros, a National League group between 1962 and 2012, are the only franchises to play both the ALCS and NLCS. The Astros are the only team to have won both an NLCS (2005) and also an ALCS (2017). The Astros made four NLCS appearances before moving to the AL in 2013. Every current National League franchise has appeared in the NLCS.