A maul occurs when three or more players, including the ball carrier and at least one other player from either side, are in contact together.
What makes the maul different to the ruck is that the ball is not on the ground but in hand.
But like the ruck, the offside line is the "hindmost" foot of the last team-mate bound to the maul.
Players can only join in from behind that team-mate. Anyone who comes in from the sides will be penalised by the referee.
Players joining the maul must have their heads or shoulders no lower than their hips and must have at least one arm bound to a team-mate.
The team not in possession of the ball cannot deliberately collapse the maul. This is for safety reasons.
Penalties can also be given for attempting to drag players out of the maul.
However this can be allowed if players are legitimately dragging out members of the opposition who have ended up on the wrong side.
One of the infringements referees have clamped down on in the past few years has been obstruction in the maul, or "truck and trailer" as it has been called.
This is when a player acts as a screen, blocking tacklers from reaching the ball carrier.
However players can circumvent this law if two or more team-mates bind around the ball together and move forwards.
As long as the tackler has a fair opportunity to contest the ball, the referee will allow the maul to continue.
'USE IT OR LOSE IT'
If the maul stops moving forwards the referee will often shout "use it or lose it" to the team in possession.
This means they must pass the ball within a five-second time period.
If they do not the referee will call a scrum and the team not in possession will be given the feed.
However if a player has caught the ball from a kick-off or a drop-out and is drawn in the middle of a maul inside their own 22m line, the referee will award the scrum to their side if the ball has not come out in time.
A maul ends when the ball is passed out or is on the ground.