<P> In rugby union a scrum is a means of restarting play after a minor infringement . It involves up to eight players from each team, known as the pack or forward pack, binding together in three rows and interlocking with the free opposing teams forwards . At this point the ball is fed into the gap between the two forward packs and they both compete for the ball to win possession . Teams can be penalised for intentionally causing the scrum to collapse, and for not putting the ball into the scrum correctly . A scrum is most commonly awarded when the ball is knocked forward, or passed forward, or when a ball becomes trapped in a ruck or maul . Because of the physical nature of scrums, injuries can occur, especially in the front row . </P> <P> To prepare for a scrum, each team's eight forwards (referred to as the pack or forward pack) bind together in three rows--the front row, second row and back row . The front row is composed of the two props and the hooker . To the left is the loosehead prop with the hooker in the middle, and the tighthead prop on the right (jersey numbers one, two and three). The "tighthead" prop, number three, earns that name as his head and shoulders sits between the hooker and the loosehead prop of the opposing side, meaning he sits in a "tighter" scrum position . The loosehead prop, by contrast, has one shoulder open to the outside of the scrum, and so is in a "looser" scrum position . The three bind together tightly and there is no gap between the hooker and either prop . According to the laws of the game; "When a player binds on a team - mate that player must use the whole arm from hand to shoulder to grasp the team - mate's body at or below the level of the armpit ." Failing to bind properly results in a penalty, and the opposition team being awarded a free kick . </P>

When do you have a scrum in rugby union