Before 1969, the National League winner (the”pennant winner”) was decided by the best win-loss record at the conclusion of the regular season. You will find four ad hoc three-game playoff series as a result of 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 broken up into two branches each, East and West. The two division winners within each league played in a best-of-five show to determine who would advance to the World Series. In 1985, the format changed to best-of-seven.
The NLCS and ALCS, since the expansion to seven games, are always played at a 2–3–2 format: matches 1, 2, 6, and 7 have been played at the stadium of the team that has home field advantage, and matches 3, 4, and 5 have been played at the arena of the group that does not. Home field advantage is given to the team that has 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 no matter regular season record and from 1995 to 1997 home field edge was predetermined before the season.
In 1981, a divisional series was held because of a split season caused by a players’ strike.
Back in 1994, the team was divided into three divisions, together with the three division winners and a wild-card team progressing to a best-of-five postseason round, the National League Division Series (NLDS). The winners of that round progress to the best-of-seven NLCS.
The Milwaukee Brewers, an American League team between 1969 and 1997, and the Houston Astros, a National League team between 1962 and 2012, would be the only franchises to play both the ALCS and NLCS. The Astros are the only team to have won both an NLCS (2005) and an ALCS (2017). The Astros made four NLCS looks before moving to the AL in 2013. Every current National League franchise has appeared in the NLCS.