Read the beginning of Chapter 2, "The Rules", from Football's Last Iron Men: 1934, Yale vs. Princeton, and One Stunning Upset by Norman L. Macht:

"In order to appreciate the events and achievements described in this narrative, it is essential to understand the rules of football then in effect. Like baseball, the sport has changed little enough for someone sitting in Palmer Stadium in November 1934 to awaken after a seventy-five-year nap and still understand what was going on in the latest Super Bowl. It has also changed so much that a twenty-first-century fan, whisked back in time to that day in Princeton, would wonder why they did the things they did the way they did them.

The most striking thing a visitor from the future would notice is the lack of substitutes. Nobody seemed to leave the game, even when the ball changed hands. Actually a coach could send in an entirely new team at any time—and Fritz Crisler often did. But any players who came out could not go back in until the next quarter. (Until 1932 they couldn’t go back in until the next half.) That slowed the in-and-out traffic considerably. It meant that a man had to be strong on both offense and defense just to make the team. Each player had strengths and weaknesses, but the specialist was unknown. If a team had no dependable kicker who could handle the other responsibilities of a back, it usually ran a play for the point after touchdown. Either way, it counted for only one point.

It was not unusual for one or two or even seven or eight men to play the entire sixty minutes. In 1926 Brown had played Yale and Dartmouth on successive Saturdays using only the same eleven men in both games.

A less visible but equally significant difference was this: the game belonged to the players. The quarterback called all the plays. He was the field commander. He could see the general staff sitting on the bench, but there could be no communication between them—no intercom, no sending in plays via messenger subs, no surreptitious nods or crossed arms or other signaling gestures, no conferring during a time-out. A violation drew a 5-yard penalty. In his first game as a coach at Yale, against Columbia, Greasy Neale suspected that the Lions’ coach, Lou Little, was giving illegal signs from the bench through a coach wearing white gloves, but he couldn’t prove it. Greasy Neale and Fritz Crisler could design all the trick plays they wanted, but it was up to the quarterback to decide when to use them. This put a tremendous burden on the quarterback and the coach who had to prepare him. It was Greasy Neale’s responsibility at Yale, and we’ll see how he carried it out."