In American and Canadian football the line of scrimmage is the imaginary transverse line crossing the football field across its narrower dimension, which passes through the last position of the football, after the end of the most recent play and following the assessment of any penalty yards. It is the line from which the next play will begin.
When the players of both teams line up to begin the next play, no member of either team may, prior to the snap of the ball, cross the line of scrimmage. In order for there to be a legal beginning of a play, a certain number of the players on the offensive team, including certain eligible receivers, must be at, on or within a few inches of the line of scrimmage.
In Canadian football the team on defence must line up no nearer than a yard to the line of scrimmage. in American football they must only be beyond the line.