The League Championship Series was created in 1969, when both the National League and the American League increased in size from ten teams to twelve with the addition, via expansion, of the Montreal Expos and San Diego Padres to the former and the Kansas City Royals and Seattle Pilots (now the Milwaukee Brewers of the NL) to the latter.
Both leagues then formed Eastern and Western Divisions, the first-place teams from which faced off in the LCS.
It also allowed those teams the unusual luxury of starting a series at home, possibly having home-field advantage in a three-game series, and a guarantee that they play two games at home.
Until 1998, the home-field advantage in the LCS was allocated on a rotating basis between the two (three from 1995 through 1997) division champions; since 1998, that advantage is given to the team with the better regular season record, except that if a division champion faces a wild card team, the division champion always gets home-field advantage regardless of record.
The Houston Astros and Milwaukee Brewers are the only teams to have played in both the ALCS and NLCS.