One of the most injurious penalties in the NFL is clipping. In most cases, players are not allowed to hit a player from behind below the waist. However, if the play is close line play, which is the area between the positions normally occupied by the offensive tackles, extending three yards on each side of the line of scrimmage. In close line play, it is legal to clip above the knee.
Clipping has the potential to cause a variety of injuries, some of them career ending, because the player being clipped has no time to prepare for the hit. In the case of close line play, clipping is allowed because players on both sides of the line of scrimmage are fighting for position against each other. Clipping is allowed in this area because it is a useful tactic for blocking an oncoming pass rusher, or pursuing a block after falling to the ground.