How does a scrum work in rugby union?

• Eight: when both teams have 15 players, eight from each bind together: two props and a hooker in front, two locks behind and three back-row players.
• A team short: may reduce its scrum in step, but "a team must not have fewer than five players in the scrum".
• 30 seconds: "Teams must be ready to form the scrum within 30 seconds of the mark being made." Free-kick.
• When: after a knock forward or throw forward or an unplayable ruck or maul. A team may also choose one instead of a penalty.
• The calls: "crouch", "bind" and "set", each when both sides are "square, stable and stationary".
• The throw: from outside the tunnel, without delay, straight and at a quick speed, so that it first touches the ground inside the tunnel.
• Pushing: only once the ball has left the scrum-half's hands, straight and parallel to the ground.
• Striking: front-row players may strike for the ball once it touches the ground. The hooker of the team throwing in must strike.
• Feet only: players in the scrum may play the ball only with feet or lower legs.
• Use it: once the ball has been available at the back for three to five seconds, the referee calls "use it".
• Offside: players not in the scrum stay at least five metres behind the hindmost foot of their team.
• It ends: when the ball comes out other than through the tunnel, when the hindmost player picks it up or the scrum-half plays it, or when it is on or over the try line.
• Reset: if it collapses or wheels past 90 degrees, and if the ball comes out at either end of the tunnel.

The glossary has the scrum, the binding entry, the hooker, the loose-head prop, the scrum-half and the uncontested scrum.

The scrum is a contest for the ball that gathers all sixteen forwards in one place and leaves the backs space to attack from it. The step-by-step engagement exists for safety: the old free charge of two front rows was dangerous, and the calls make the packs come together slowly and square.