Volleyball in : The libero

The object is simple to state and hard to execute: ground the ball inside the opponent's 9-by-9-metre court, or force them into an error, before they can do the same to you. A team may contact the ball three times - conventionally a dig or pass, a set, and an attack - and must send it back over the net without catching, carrying or hitting it twice in a row. A block does not count as one of the three touches. Every rally ends in a point for one side; there are no ties and no clock.

A match is a race to win three sets. The first four sets are played to 25 points and the deciding fifth to 15, each with a two-point margin required, so a set can stretch well beyond its target - 28-26, 30-28 - until one side leads by two. Teams rotate one position clockwise each time they win back the serve, so every player serves, defends the back row and attacks the front in turn, subject to substitution and the libero exception.

A team may touch the ball a maximum of three times before it must cross the net, and the same player may not make two consecutive contacts. The classic sequence is pass or dig, then set, then attack. A team block is the exception - it does not count as one of the three team touches, so the same player who blocks may make the next contact. Letting the ball hit the floor, sending it out of bounds, or needing a fourth touch all end the rally in the opponent's favour.