The scrum is a means of restarting play after a stoppage which has been caused by a minor infringement of the Laws (for example, a forward pass or knock-on) or the ball becoming unplayable in a ruck or maul. The scrum serves to concentrate all the forwards and the scrum-halves in one place on the field, providing the opportunity for the backs to mount an attack using the space created elsewhere.
In 15-a-side rugby there are eight players in the scrum on each team and in rugby sevens each side has three. The ball is thrown into the middle of the tunnel between the two front rows, at which point the two hookers can compete for the ball, attempting to hook the ball back in the direction of their team-mates. The team who throws the ball into the scrum usually retains possession, because the hooker and scrum-half can synchronise their actions.
Once possession has been secured, a team can keep the ball on the ground and in the scrum and attempt to drive the opposition down field. 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.
This player 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.
As soon as the scrum-half picks the ball up, the opposition may contest for the ball and attempt to tackle whichever player is in possession.