On September 15, 1928, the Boston Braves played their ninth consecutive doubleheader. During the streak, which began on September 4th, the Braves lost five in a row, including four to the Giants.
This day in baseball: Doubleheader streak
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They probably threw three starters in that streak, and those guys went the distance seven or eight times.
I wouldn’t doubt it. Pitchers were made of tougher stuff back then.