The American League and National League reach an agreement on next year’s expansion format. The twelve teams in each league will be divided into two divisions playing a best-of-five games League Championship Series to determine the pennant winner.

On July 10, 1968 The American League and National League reach an agreement on next year’s expansion format. The twelve teams in each league will be divided into two divisions playing a best-of-five games League Championship Series to determine the pennant winner.

Source

Baseball Reference July 10