Before 1969, the National League winner (the”pennant winner”) was determined by the best win-loss record in the conclusion of their regular season. You will find 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, but utilized a single-game playoff.)
A structured postseason show began in 1969, when both the National and American Leagues were broken up into two branches each, East and West. The two division winners within each league played each other 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, since the expansion to seven games, are always played in a 2–3–2 format: matches 1, 2, 6, and 7 are played at the stadium of the group that has home field advantage, and matches 3, 4, and 5 have been played in the stadium of the group that doesn’t. Home field advantage is given to the team with the better record, with the exception that the team that produced the postseason since 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 edge was specified prior to the year.
Back in 1981, a divisional show was held due to a split season brought on by a players’ strike.
In 1994, the team was divided into three branches, with the three division winners and a wild-card team progressing 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, also a National League group between 1962 and 2012, are the sole franchises to play both the ALCS and NLCS. The Astros will be the only team to have won both an NLCS (2005) and also an ALCS (2017). The Astros created four NLCS appearances before moving to the AL in 2013. Every current National League franchise has emerged in the NLCS.