The history of American football can be traced to early versions of rugby football and association football. Both games have their origin in multiple varieties of football played in the United Kingdom in the mid-19th century, in which a football is kicked at a goal or kicked over a line, which in turn were based on the varieties of English public school football games descending from medieval ball games. American football resulted from several major divergences from association football and rugby football. Most notably the rule changes were instituted by Walter Camp, a Yale University athlete and coach who is considered to be the "Father of American Football". Among these important changes were the introduction of the line of scrimmage, of down-and-distance rules, and of the legalization of forward pass and blocking. In the late 19th and early 20th centuries, gameplay developments by college coaches such as Eddie Cochems, Amos Alonzo Stagg, Parke H. Davis, Knute Rockne, and Glenn "Pop" Warner helped take advantage of the newly introduced forward pass. The popularity of college football grew as it became the dominant version of the sport in the United States for the first half of the 20th century. Bowl games, a college football tradition, attracted a national audience for college teams. Boosted by fierce rivalries and colorful traditions, college football still holds widespread appeal in the United States. The origin of professional football can be traced back to 1892, with Pudge Heffelfinger's $500 ($ in dollars) contract to play in a game for the Allegheny Athletic Association against the Pittsburgh Athletic Club. In 1920 the American Professional Football Association was formed. This league changed its name to the National Football League (NFL) two years later, and eventually became the major league of American football. Primarily a sport of Midwestern industrial towns in the United States, professional football eventually became a national phenomenon. The modern era of American football can be considered to have begun after the 1932 NFL Playoff game, which was the first indoor championship game since 1902 and the first American football game to feature hash marks, forward passes anywhere behind the line of scrimmage, and the movement of the goalposts back to the goal line. Other innovations to occur immediately after 1932 were the introduction of the AP Poll in 1934, the tapering of the ends of the football in 1934, the awarding of the first Heisman Trophy