THE purpose of a scrum is to restart play after a minor infringement such as a knock-on, throw forward or a lineout infringement such as the ball not being thrown in straight.
These days, the rules dictate that there are usually eight players from each team in the scrum, other than when a player has been sent off or a team has fewer than 15 players.
A scrum can only take place in the field of play, but not less than five metres from a goal line or a touch line.
All players must be bound firmly and continuously until the scrum is over, and the hooker must bind either over or under his props' arms.
Props bind with one arm around the hooker and the other on the opposing prop.
The loosehead prop binds inside the tighthead's arm and may grip the tighthead's jersey, either on the back or the side.
He may not grip the chest, arm, sleeve or collar, nor can he exert downward pressure on his opponent.
Tightheads obviously bind their opponent likewise, with their arm outside the loosehead's arm.
These bindings can be altered (legally) during the scrum.
Props can no longer place one hand on their thigh, as they were allowed to a few years ago.
Flankers bind on the locks and must not move out to obstruct the opposing halfback as he follows the ball around the scrum.
While the flanker may pack at an angle to make it more difficult for the halfback, that doesn't help the shove.
The rules of engagement have changed with monotonous regularity in recent years to make the scrum safer for players, especially the front row.
Currently, the sequence goes like this:
When the referee can see all players are in position, the call is made to "crouch".
When the front rows are in a suitably crouched position, with heads looking up, the call is "bind". This is the signal for the props to bind on their opponent and place their heads in an "ear-to-ear" position, without actually engaging shoulders.
When the referee can see the scrum is stable, with no weight being pushed forward, he will give the call to "set", which is the call for the front rows to engage.
At this point, there should be no shoving opponents backwards, early or otherwise.
Before allowing the halfback to put the ball in, the referee is looking to make sure that the scrum is square and stable.
Before the scrum sequence begins, the referee makes a mark which is where the "middle line" of the scrum will be formed at the junction of the props' shoulders after engagement.
The halfback must stand one metre from the scrum and put the ball in without delay when requested by the referee, usually by a pat on the back or a hand signal from the other side of the scrum.
The ball must be put in straight along the middle line so that at least part of it touches that imaginary line, and not at an angle so that it arrives under his hooker's feet.
This is a point of difference from past seasons, when the middle of the ball had to go along the middle line. You be the judge when watching rugby on television.
Play in the scrum officially begins when the halfback puts the ball in and the two packs strive for possession by pushing straight ahead and trying to hook the ball.
If only it was that simple.
Once the ball goes into the scrum, the contest to win the ball begins, and that will be the subject of next week's column.