Definitions

The purpose of the scrum is to restart play quickly, safely and fairly, after a minor infringement or a stoppage.
A scrum is formed in the field of play when eight players from each team, bound together in three rows for each team, close up with their opponents so that the heads of the front rows are interlocked. This creates a tunnel into which a scrum half throws in the ball so that front row players can compete for possession by hooking the ball with either of their feet.
The middle line of a scrum must not be within 5 metres of the goal line. A scrum cannot take place within 5 metres of a touchline.
The tunnel is the space between the two front rows.
The player of either team who throws the ball into the scrum is the scrum half.
The middle line is an imaginary line on the ground in the tunnel beneath the line where the shoulders of the two front rows meet.
The middle player in each front row is the hooker.
The players on either side of the hooker are the props. The left side props are the loose head props. The right side props are the tight head props.
The two players in the second row who push on the props and the hooker are the locks.
The outside players who bind onto the second or third row are the flankers.
The player in the third row who usually pushes on both locks is the No.8. Alternatively, the No. 8 may push on a lock and a flanker.