American football is a purely American sport which has become by far the most popular spectator sport in the United States, both on the professional and college level. It was originally developed in the late 1800s as a variant of rugby football, but has evolved greatly. The major difference between rugby football and American football is that American football allows the use of the forward pass to advance the ball. The only time the sport has been demonstrated at the Olympics was in 1932 when two college all-star teams played, one representing the East (players from Harvard, Yale, and Princeton), and one representing the West (players from Cal Berkeley, Stanford, and USC).