In rugby union, a scrum is a means of restarting play after a minor infringement, such as a forward pass or knock-on, or when the ball becomes unplayable in a ruck or maul). The scrum involves up to eight players from each team, known as the pack or forward pack, binding together in three rows and pushing against each other). The scrum serves to concentrate all the forwards and the scrum-halves in one place on the field, providing the opportunity for the backs to mount an attack using the space created elsewhere.

Here are some key points about the scrum in rugby:

• In 15-a-side rugby, there are eight players in the scrum on each team, and in rugby sevens, each side has three.
• The ball is thrown into the middle of the tunnel between the two front rows, at which point the two hookers can compete for the ball, attempting to hook the ball back in the direction of their teammates.
• Once possession has been secured, a team can keep the ball on the ground and in the scrum and attempt to drive the opposition downfield. Alternatively, they can bring the ball to the hindmost foot of the scrum, where the ball is then passed into the backline and open play resumes again.
• The key player at the scrum is the scrum-half. This player throws the ball into the scrum, moves to the hindmost foot of the scrum, and is usually the player who picks the ball up and passes out to the fly-half who then distributes the ball to the backline.
• Teams can be penalized for intentionally causing the scrum to collapse, and for not putting the ball into the scrum correctly).
• There are a large number of rules regarding the specifics of what can and cannot be done during a scrum. This is mainly for safety reasons, because if a scrum is not conducted properly, there is a greater chance of injury).
• Front rowers must engage square on, rather than bore in on an angle. The tight-head prop doing this limits the movement of the opposing hooker. Scrums are one of the most dangerous phases in rugby, since a collapse or improper engage can lead to a front row player damaging or even breaking their neck. For this reason, only trained players may play in the front row to help avoid injuries).
• A scrum is formed in the scrum zone at a mark indicated by the referee. The referee makes the mark to create the middle line of the scrum, which runs parallel to the goal lines. Teams must be ready to form the scrum within 30 seconds of the mark being made.

In summary, a scrum is a way to restart play after a minor infringement in rugby union. It involves up to eight players from each team pushing against each other to gain possession of the ball. There are many rules and safety precautions in place to ensure that scrums are conducted properly and safely.