in a formation, you must have both of your tackles (end linemen) "covered". this means having a receiver or tight end on the line. no more, and no less than 7 men may be on the line. every other player must be at least 1 yard behind the line of scrimmage (in nfl rules). you may have multiple wr's on one side of the line (trips formation), but only one is allowed on the line of sscrimmage.
confused yet?
