In rugby league, a scrum is used to bring the ball back into play in situations where the ball has gone out of play over the touchline or a player has made a mistake, such as a knock-on or forward pass, except when that mistake has occurred on the last tackle of a set of six tackles). The scrum is also used in the rare event that the ball bursts or the referee interferes with the movement of the ball). The main purpose of the scrum is to restart play and provide an opportunity for the backs to mount an attack using the space created elsewhere on the field. The scrum serves to concentrate all the forwards and the scrum-halves in one place on the field, providing this opportunity for the backs. Once possession has been secured, a team can keep the ball on the ground and in the scrum and attempt to drive the opposition downfield. Alternatively, they can bring the ball to the hindmost foot of the scrum, where the ball is then passed into the backline and open play resumes again. The key player at the scrum is the scrum-half, who throws the ball into the scrum, moves to the hindmost foot of the scrum, and is usually the player who picks the ball up and passes out to the fly-half who then distributes the ball to the backline.