How did the 1969 expansion change the structure of the American and National leagues?

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Barry Bonds of the San Francisco Giants swings his bat after hitting his record-breaking 756th home run, with the catcher, umpire, and a cheering crowd behind him at AT&T Park. — History of baseball Giants legend Barry Bonds hitting his record-setting 756th career home run, August 7, 2007.

In 1969, the American League and National League underwent significant structural changes due to league expansion.

Both leagues expanded to 12 teams each by adding new franchises:
* National League: Added teams in Montreal (the Expos) and San Diego (the Padres).12
* American League: Added teams in Kansas City, Missouri (the Royals) and Seattle, Washington (the Pilots).12

To accommodate this growth, both leagues were divided into Eastern and Western divisions. This structural shift introduced a new postseason format where the winners of these divisions competed in playoffs to determine the league pennant winners, who then advanced to the World Series.12