PLAY begins in the scrum when the ball leaves the halfback's hands. Both teams can then push, while any front-row player (and only them) can strike in an attempt to win the ball.
With the current engagement sequence lessening the chances of any one team gaining an advantage on the "hit", front-row players, principally hookers, often find it difficult to hook the ball as the pressure is so great.
This is the main reason for a recent change in rulings allowing the halfback to put the ball in straight along the "middle line", so that any part of it touches that imaginary line.
Previously, the middle of the ball had to cross the middle line.
However, the halfback still cannot put the ball in at an angle so that it arrives under his hooker's feet.
During the scrum, front-row players may not deliberately twist, dip their body or collapse the scrum.
Nor can they lift or drive an opponent up in the air. These actions are considered dangerous play.
It can sometimes be difficult for the referee to tell what actions, and by which players, are responsible for collapsing scrums.
There was a story some years ago where Andrew Fordice, an Australian test referee, was drinking with props on opposing teams the night before a club game.
They argued all referees only "guessed" which players were deliberately collapsing scrums and suggested they would put him to the test by conspiring to do it in the game next day.
After the game, they told him, "congratulations, you got seven out of 10 ... wrong".
Once the ball is hooked, any other player may use his feet to win the ball.
They cannot return the ball to the scrum if it comes out "disadvantageously" for them, nor can the halfback kick the ball while it is in the scrum.
If the ball comes out either end of the tunnel, the scrum is reset.
A team is allowed to wheel a scrum, but only after an initial straight push.
If the scrum wheels more than 90 degrees it has to be reset, with the team not in possession at the time putting the ball in at the new scrum.
For under-19 grades and below - mainly schoolboy rugby - the rules for wheeled scrums are changed somewhat.
In these grades a team is not allowed to deliberately wheel the scrum at all.
If the scrum does wheel accidentally, the referee blows his whistle once it has passed 45 degrees and resets the scrum, with the original team putting the ball in again.
Also, in these grades teams are not allowed to push more than 1.5m and must release the ball immediately when it reaches the back of the scrum.
A scrum ends successfully when one of three things happen.
First, it comes out legally and is picked up by a player, usually the halfback, and play continues.
Or the No8 unbinds with the ball beneath him and picks the ball up and moves away or passes it.
And a scrum also ends when the ball crosses the goal-line and any on-side player can dive on it, even if the ball is still in the confines of the scrum.
There are different offside lines for the halfbacks and the other backs.
The halfback of the team putting the ball into the scrum must not remain with both feet in front of the ball - one foot is all right - otherwise he is offside.
The opposing halfback must stand beside his opponent before the ball is put in, or retire behind the 5m line with the other backs.
If he stays beside the other scrumhalf he may follow the ball as it travels through the opposition forwards' legs, and may have one foot in front of it, but not both.
He may also retire to the hindmost foot of his players' scrum, but not advance on the other side until the ball comes out.
If he does retire to this line he must remain close (within 1m) of the scrum and not move further out in order to tackle opposing players.
One other way he can be offside is to place his hand on, or grasp, a player in the opposing scrum - halfbacks tend to be devious people and this can be the cause of friction with opposing flankers.
The remaining backs must remain 5m behind the hindmost foot of players in their scrum until the ball is out.
There is another variation for the under-19 and below grades.
Scrum numbers must be matched so if a team has a player in the sinbin or sent off, both teams must have seven players in the scrum.
Likewise, if a team has less than a full team, scrum numbers must be matched accordingly.
Uncontested scrums can be called for when a team does not have three suitably trained front rowers.
In these scrums there is no pushing and the team putting the ball in must win the ball.
Such scrums, for some reason or other, are very unpopular with the forwards, especially the front-row ones. They do make refereeing the scrum easier, though!