Before 1969, the National League champion (the”pennant winner”) was determined by the very best win-loss record in the conclusion of the regular season. There were four ad hoc three-game playoff series due to ties under this formula (in 1946, 1951, 1959, and 1962). (The American League needed to solve a tie in 1948, but used a single-game playoff.)
A structured postseason show started in 1969, when both the National and American Leagues were broken up 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 structure changed to best-of-seven.
The NLCS and ALCS, because the expansion to seven games, are played in a 2–3–2 format: games 1, 2, 6, and 7 have been played at the arena of the team that has home field advantage, and matches 3, 4, and 5 have been played at the arena of the group that doesn’t. 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 divisions each year regardless of regular season record and from 1995 to 1997 home field edge was predetermined before the year.
In 1981, a divisional series was held because of a split season caused by a players’ strike.
Back in 1994, the league was restructured into three divisions, together with the three division winners and a wild-card team advancing to a best-of-five postseason round, 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, also a National League team between 1962 and 2012, would be the only franchises to play both the ALCS and NLCS. The Astros will be the only team to have won both an NLCS (2005) and an ALCS (2017). The Astros made four NLCS appearances before going to the AL in 2013. Every current National League franchise has emerged in the NLCS.