Scrum (rugby)
In rugby football a scrum is a way of restarting the game, either after a minor infringement (in rugby union, a set scrum), or when the ball has gone onto the ground after a successful tackle (in rugby union only, and known as a loose scrum or ruck). Scrums occur more often, and are of greater importance, in union than in league.
Before a scrum is formed the eight forwards from each team bind together in three rows (three players in the front row, two in the second row, and the remaining players behind them). The front row is composed of the two prop forwards (tight head and loose head) supporting the hooker; the second row forwards are more often referred to as locks; and the back row is made up of the two flankers (sometimes wing forwards) with the Number 8 (named after the jersey number of the starter at that position) between them.
Hookers are normally the smallest members of the scrum. Props and locks both need to be strong and tall, but the positions differ in their main criteria for selection. Since props are more directly involved in the push forward, strength is more important for them than height. Strength is also important for locks, since they also push; however, height is more important for them. Locks are virtually always the tallest players on the team; they are used as the primary contestants for possession in another phase of the game, the line-out. Flankers and the Number 8 do less of the pushing in the scrum, and need more speed, because their task is to break quickly and cover the opposing half-backs if the opponents win the scrum.
The two packs of forwards approach to within a short distance of each other and crouch. On a signal from the referee the front rows engage with each other
so that their heads are interlocked with those of the other side's front row. Another player (the scrum half) from the team that did not infringe then throws the ball into the tunnel thus formed, and the front row players compete for possession by trying to hook the ball backwards with their feet, while the entire pack tries to push the opposing pack backwards. The side that wins possession usually transfers the ball to the back of the scrum, where it is picked up by its scrum half, who will either pass it out to the fly half and the other backs, or kick ahead over the heads of the scrum, then running forward to put his or her forwards onside. On other occasions the forwards will hold the ball in the scrum and try to push the opposition backwards.
It is the scrum, and also the line-out, that gives rise to the simplified explanation of rugby union: "The forwards are there to get the ball back, and the backs are there to get the ball forward".The Rugby Union Scrummage