In both sports, a scrum is formed by the players who are designated forwards binding together in three rows. The scrum then 'engages' with the opposition team so that the player's heads are interlocked with those of the other side's front row. The scrum half from the team that did not infringe then throws the ball into the tunnel created in the space between the two sets of front rowers' legs. Both teams may then try to compete for the ball by trying to hook the ball backwards with their feet.
A key difference between the two sports is that in rugby union both sets of forwards try to push the opposition backwards whilst competing for the ball and thus the team that did not throw the ball into the scrum have some chance of winning the possession. In practice, however, the team with the 'put-in' usually keeps possession. Forwards in rugby league do not push in the scrum, often feed the ball directly under the legs of their own front row rather than into the tunnel, and the team with the put-in almost always retains possession.
The word "scrummage" is a modification of "scrimmage" (the form of the word previously used in rugby and still used in American and Canadian football), which in turn derives from or is a cognate of "skirmish". The term was used in the laws of rugby union for a long time before being permanently contracted to just "scrum". Scrum has caused many injuries to many players and there have been two reported deaths from injuries in scrums.
Originally there was no distinction between an awarded or "set" scrum (today officially called simply "scrummage") and a "loose" scrum (today officially called a ruck). The side awarded a scrimmage simply had one player put the ball on the ground and let go of it; there was no requirement of a tunnel, although players were required to be onside, i.e. not ahead of the ball. The most common way for a scrimmage/scrummage to be so awarded (there being no referee to actually award one, but as the rules specified) would be the occurrence of a stalemate between the player with the ball (who would declare "held") and opponents holding him (who would call, "Have it down"). A scrummage could also occur as a ruck today, in which opposing players simply close around a ball already on the ground.
Although the rules of playing the ball were different as to whether it was in scrimmage or not, the early rules did not draw a clear distinction between players in or out of scrimmage, and did not require players in scrimmage to bind. Early accounts of play show that in fact they could not have been bound, for they would try to work their way through the pack while attempting to get to and dribble the ball.
The early rules of rugby, even after recodification as "Laws of the Rugby Union" (the term "laws" having been borrowed from the Football Association), said the object of players in the scrummage was to kick the ball towards their opponents' goal line. This provision remained in the laws for approximately 20 years after practice had changed in the late 19th century.
The modern scrummage and ruck, the rugby league play-the-ball, and the American football snap and scrimmage (later adopted by Canadian football) were all derivatives of the early scrummage, and responsive in different ways to problems encountered in the way the rules regarding it were written and administered.
A rugby union scrum consists of two team's eight forwards, with each team binding in three rows. The front row is composed of the two s and the . The two second row forwards (jersey numbers four and five), commonly referred to as the s bind together and directly behind the front row with each putting their heads between the props and the hooker. Lastly the back row is made up of the two s and the . The flankers bind on each side of the scrum — next to a lock and behind a prop.
The two forward packs form a scrum by approaching to within an arms length of each other. The referee gives the command crouch and the opposing front rows then crouch. Then the referee calls touch and props touch the opposites outside shoulder. The referee then issues the pause command to inspect the scrum, and lastly engage and the two front rows come together. When this happens both front rows thrust forward with the tighthead props' heads going between the opposing hooker and loosehead prop. The props then bind by gripping the back or side of the opposing prop's jersey. The from the team that has possession then throws the ball in the gap formed between the two front rows. The two hookers (and sometimes the props) then 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 — which is done with their feet. Once at the back it is picked up either by the number 8, or by the scrum-half.
There are a large number of rules regarding the specifics of what can and cannot be done during a scrum. Front rowers must engage square on, rather than bore in on an angle. Front-rowers are also banned from twisting their bodies, pulling opponents, or doing anything that might collapse the scrum. The back row must remain bound until the ball has left the scrum. For flankers, this means keeping one arm, up to the shoulder, in contact with the scrum. The scrum must be stable, stationary and parallel to the goal-lines when they feed the ball; otherwise a free kick is awarded to the non-offending team. The ball must be fed into the middle of the tunnel with its major axis parallel to the ground and touchline. The ball must be thrown in quickly and in a single movement — this means that a feed cannot be faked. Once the ball has left the hands of the scrum-half the scrum has begun.