The three major rules of play in ice hockey that limit the movement of the puck: "offside", "icing", and the puck going out of play. A player is offside if he enters his opponent's zone before the puck itself. Under many situations, a player may not "ice the puck", which means shooting the puck all the way across both the centre line and the opponent's goal line. The puck goes out of play whenever it goes past the perimeter of the ice rink (onto the player benches, over the glass, or onto the protective netting above the glass) and a stoppage of play is called by the officials using whistles. It also does not matter if the puck comes back onto the ice surface from those areas as the puck is considered dead once it leaves the perimeter of the rink. The referee may also blow the whistle for a stoppage in play if the puck is jammed along the boards when 2 or more players are battling for the puck for a long period of time, or if the puck is stuck on the back of any of the two nets for a period of time.