The Yankees–Red Sox rivalry is a Major League Baseball (MLB) rivalry between the New York Yankees and Boston Red Sox. Both teams have competed in MLB's American League (AL) for over 100 seasons and have since developed the fiercest rivalry in all of American sports. In 1919, Red Sox owner Harry Frazee sold star player Babe Ruth to the Yankees, which was followed by an 86-year period in which the Red Sox did not win a World Series. This led to the popularization of a superstition known as the "Curse of the Bambino", which was one of the most well-known aspects of the rivalry.