Rule origin

[Jeremy Crowhurst]

Does anybody know why the rule is that the clock stops after an incomplete pass or a play out of bounds, but keeps running for plays that end within the field of play?

[Bob Gill]

The clock stops after an incomplete pass because in the old days the referees often had to run after the lone ball and bring it back to the line of scrimmage for the next play, and that could occasionally waste a lot of time. I'd guess that out-of-bounds plays sometimes caused the same kind of delays -- if the crowd was too close to the sideline, for instance -- and that accounts for the stoppage of the clock for them.

[JuggernautJ]

It would make sense to me that the rule is a logical extension of football as a territorial game.

If the ball stays in the field of play the clock continues uninterrupted.
If the ball leaves the field of play (by exiting the sidelines) or the flow of play is interrupted (by an incomplete pass) the clock is stopped until play resumes.