with three halfbacks who alternated at quarterback and a lone fullback. Princeton put six men on the line and had one designated quarterback, while Yale used seven linemen, one quarterback and two halfbacks who lined up on either side of the fullback. This was the origin of the T-formation, an offensive set that remained in use for many decades afterward and gained popularity in professional football starting in the 1930s. In 1906, the forward pass was legalized in American football; Canadian football did not adopt the forward pass until 1929. Despite the legalization of the forward pass, the most popular formations of the early 20th century focused mostly on the rushing game. The single-wing formation, a run-oriented offensive set, was invented by football coach Glenn "Pop" Warner around the year 1908. In the single-wing, the quarterback was positioned behind the line of scrimmage and was flanked by a tailback, fullback and wingback. He served largely as a blocking back; the tailback typically took the snap, either running forward with the ball or making a lateral pass to one of the other players in the backfield. The quarterback's job was usually to make blocks upfield to help the tailback or fullback gain yards. Passing plays were rare in the single-wing, an unbalanced power formation where four linemen lined up to one side of the center and two lined up to the other. The tailback was the focus of the offense, and was often a triple-threat man who would either pass, run or kick the ball. Offensive playcalling continued to focus on rushing up through the 1920s, when professional leagues began to challenge the popularity of college football. In the early days of the professional National Football League (NFL), which was founded in 1920, games were largely low-scoring affairs. Two-thirds of all games in the 1920s were shutouts, and quarterbacks/tailbacks usually passed only out of desperation. In addition to a reluctance to risk turnovers by passing, various rules existed that limited the effectiveness of the forward pass: passers were required to drop back five yards behind the line of scrimmage before they could attempt a pass, and incomplete passes in the end zone resulted in a change of possession and a touchback. Additionally, the rules required the ball to be snapped from the location on the field where it was ruled dead; if a play ended with a player going out of