For more than 60 years, baseball had been limited in its number of franchises. As times changed in America and the game increased in popularity, other cities saw opportunities that a baseball franchise could provide.
In 1962, the National League expanded, bringing the Colf 45s (Astros) to Houston and the Mets to New York. In 1969, both leagues expanded their lineups to 12 teams. The NL brought teams to San Diego and Montreal. The AL brought teams to Kansas City and Seattle. The Seattle Pilots only lasted a year in the town and moved to Milwaukee to become the Brewers in 1970.
The structure of the leagues also changed during this period. Both the NL and the AL went to two divisions of six teams each. The change also brought a modification to the playoffs . Now, the two division winners squared off in a 5-game series to become the League Champ and the representative in the World Series.
New stadiums arrived on the scene, bringing the distance to the outfields in and creating more offense in the league. Older parks had some facelifts but remained crucial parts of the game.
Pitchers started to battle back against hitters by adding new pitches that took advantages of curves and off-speed to keep batters off-balance. The 1968 season was dubbed the year of the pitcher -- one in which a modification to the mound gave pitchers unprecedented advantage over the batter. That however was changed the next season.