A designation for any player who begins a play outside of the field of play. The field of play is marked by the white line along the side of the field. Once play has begun, any player who was out of bounds at the snap may not participate in the play on the field. If a player who began the play out of bounds affects the play, it is a five-yard penalty on the offending team, and the down is replayed. Also, since a team may only have 11 players on the field of play, any extra players must reach the out of bounds area prior to the ball being snapped, or a penalty is incurred.
Out of bounds is the easiest way to evaluate who will participate in any given play. The 11 players on each team must be clearly in bounds and not in contact with the boundary in any way. Additionally, any players substituting out must reach the out of bounds area prior to the snap. Out of bounds penalties are usually assessed as five yards and a replayed down.