In gridiron football, clipping is the act of a "throwing the body across the back of the leg of an eligible receiver or charging or falling into the back of an opponent below the waist after approaching him from behind, provided the opponent is not a runner." It is also clipping to roll up on the legs of an opponent after a block. It is usually illegal, but in the National Football League it is legal to clip above the knee in . The Canadian Football League has similar definitions, prohibitions and exceptions, including that "application of [a] penalty is determined by the initial contact".