Before scrimmage, an official places the ball at the spot it was at the stop of clock, but no nearer than 24 yards from the sideline or 1 yard from the goal line. The line parallel to the goal line passing through the ball (line from sideline to sideline for the length of the ball) is referred to as the line of scrimmage. This line is similar to "no-man's land"; players must stay on their respective sides of this line until the play has begun again. For a scrimmage to be valid the team in possession of the football must have seven players, excluding the quarterback, within one yard of the line of scrimmage. The defending team must stay a yard or more back from the line of scrimmage.
How close to the length of the ball can defensive players be before a play?