Motion (gridiron football)
Motion and shift 
There is a distinction drawn between a shift and motion in football. A shift occurs when one or more players changes their position on the offensive side of the ball before the snap, causing a change in formation. For example, players may line up initially in an I-formation and then shift the two running backs into wide receiver positions to put the offense in a spread formation. A team may shift any number of players into new positions, so long as they all come to a complete stop for a full second before the ball is snapped to start the play.
Motion occurs when a player is moving at the time of the snap. While different leagues have different rules regarding motion, most mandate that no more than one player may be in motion at the time of the snap, and that only players who start in "back" positions (running backs, fullbacks, quarterbacks, flankers, H-backs, etc.) may be in motion at the snap. Additionally, the NFL (professional), NCAA (college), and NFHSAA (high school) require that they must be moving laterally or backwards, and are not allowed to move towards the line of scrimmage. The Canadian Football League and the Arena Football League allow for motion towards the line of scrimmage.
The National Football League defines all motion and shift penalties as "illegal motion", while both the NCAA and NFHSAA make a distinction between an "illegal shift" and "illegal motion"; an illegal shift refers to players shifting and not coming to a complete stop before the snap while illegal motion refers to a player who is in motion towards the line of scrimmage, or a player who is not a "back" in motion. In both leagues, however, the penalty for illegal motion/illegal shift is five yards from the previous spot and replay the down.
Additionally, the offensive team may be charged with the penalty of a "false start" if a player on the offense jumps or moves abruptly, simulating the start of the play. This movement is not normally considered a subset of the "motion" or "shift" rules, as the player is not judged to be moving into a new pre-snap position; they are merely starting the play too soon. This is also a five yard penalty.
History and purpose 
In the earliest days of American football, offenses were allowed to shift and assemble themselves as much as they wanted, much as defenses do. The famous Notre Dame Box relied heavily on these shifting motions. However, rule changes were eventually implemented that prevented offenses like the Notre Dame Box from ever occurring again. The motion rules seen today resulted from these rule changes. Currently, wholesale formation shifts can only occur before the offensive formation is set, and said formation must be in place for at least one second before a snap or motion can occur.
The purposes of motion are to allow the offense to change formations and, in leagues that allow forward motion, a chance to gain momentum on the defenders.
In all forms of football, only players in the backfield and not on the line of scrimmage may be in motion at the time of the snap. Prior to starting the motion, all players on the offensive side must be in a set formation for a minimum of one second.
In most versions of American football, only one player may be in motion at one time, and the player must not move toward the line of scrimmage in his motion (in other words, he can only move laterally or backward). In no situation may the moving player begin on the line of scrimmage when he moves (in other words, offensive linemen are prohibited from motion prior to the snap). Any player who shifts from a lineman position to a back position must set in position at least five yards behind the line of scrimmage before going into motion.
Exceptions are as follows:
- Pro Bowl: Motion is prohibited.
- XFL, WFL, Arena Football League and most other indoor football leagues: One player in motion is allowed; he can move toward the line of scrimmage only if he is outside the offensive tackles.
- American Indoor Football: Two players in motion are allowed, both can move toward the line of scrimmage outside the tackles.
- Canadian football: All backfield players are allowed to move toward the line of scrimmage at the snap, regardless of their horizontal position.
In leagues that allow forward motion, the moving player(s) cannot cross the line of scrimmage, or else it is a false start.
See also 
- "Backfield in Motion," a 1969 song that borrows from the practice
Additional reading 
- National Federation of State High School Associations Rule Book, 2001