Play is too dangerous to continue
Not everyone can join a scrum. Only six players from each team can take part. They are almost always the six forwards in the side.
The scrum is formed at the place where the infringement occurred. However, all scrums must take place at least 10m from the touch or goal lines.
The team which did not commit the offence will have the advantage in the scrum. They will be the team with the loose head and feed.
Forming a scrum
The six forwards are the players who form the scrum.
The hooker, two props, two second rows and the loose forward all bind together in a 3-2-1 formation.
The hooker is the all important player in the scrum - it's their job to get the ball out for the scrum-half.
To make the hooker's life that bit easier are the two props.
They bind on tightly on both sides of the hooker, leaving no gaps between them.
Next comes the two second row forwards.
They bind tightly together and pack down behind the front row, putting their heads in the gaps between the hooker and the props.
The final member is the loose forward.
They pack down behind the second row forwards, putting their head in between the two second rows.
Feeding the scrum
It's the scrum half's job to feed the ball into the scrum for the hooker to strike back to the loose forward.
The scrum-half must roll the ball in from the side where the referee is standing.
From there, the scrum-half cannot handle the ball until it has come out of scrum.
The six other backs must be at least five metres behind the last forward of their team.
If they're not, the referee will penalise the offending team.
A scrum can be re-taken again if it moves quite a bit of distance from its original spot before the ball has been put in.
Hooking the ball
The hooker's job may sound easy but striking the ball back to the loose forward is not as easy as it seems.
Why? Because the opposition's hooker is trying to steal the ball from you.
Plus you've got six huge forwards on the other side trying to push you off the ball.
The hooker is the only player in the scrum who can raise their feet - otherwise they would never be able to strike the ball.
However, no player in the scrum is allowed to handle the ball in the scrum until the ball is free - not even the hooker.