Rules
The offense is required to set up a formation before a play, subject to several rules:
The formation must have at least 7 players on the line of scrimmage.[1] The 7 players are not required to be next to each other, so they may spread out across the width of the field, but this is rare; most offenses place at least 5 players together in a continuous line. Teams may place more than 7 players on the line, but only the player at each end of the line may be an eligible receiver, so generally this only occurs with the special formations used in kicking and punting situations.
The other players not on the line (usually 4 in number) may be positioned anywhere, but all must be at least 1 yard behind the 7 or more players on the line of scrimmage.[1] The traditional saying is "7 on the line, 4 in the backfield" but this is something of a misnomer, as "backfield" usually refers to the area directly behind the offensive line. 3 of the 4 "backfield" players (i.e., the 3 not receiving the snap from center) may line up as wide receivers as long as they are behind the line of scrimmage; these are known as slot receivers if between the ends, and flankers if outside the ends.
Of the 7 (or more) players on the line of scrimmage, all except those at either end of the line are ineligible receivers: these players may not touch or catch a forward pass[2] (unless it is first touched by an eligible receiver, including a deflection by a defensive player), and on a forward pass play, they may not advance downfield (i.e., across the neutral zone) before the pass crosses the neutral zone. Ineligible receivers may advance freely on a running play or after a pass is thrown. These players typically have uniform numbers in the range 50-79 to indicate they are ineligible.
