Over the course of the long Red Sox-Yankee history, a single series between the rivals has occasionally served as the defining moment of a particular season for both franchises.
In 1978, that series was the legendary Boston Massacre, when the Yankees, having already stormed back from being 14 games back in the summer, nearly finished the Sox off with a four-game sweep of the Sox at Fenway in early September. The Sox eventually had their own comeback later that month, but we all know how that ended.
In 2006, against the backdrop of a full-on Manny Ramirez tantrum, the Yankees took a five-game series from the Sox -- a four-game set grew because of a makeup game from earlier in the season -- and effectively finished them off for the year. The Sox limped to a third-place finish, 11 games behind New York.
Last year, the Red Sox used a Yankee visit to Fenway in early August to salt away their division championship. Leading by 5.5 games when the four-game set began, the Sox won all four to move 9.5 games ahead and the East was effectively theirs. The winning margin was eight at the end.