Eight teams play each of their seven rivals once before the top four teams advance to the medal rounds, in which there are semi-finals, finals and a grand final.

Competing teams take it in turns to bat and field, the object being to score the most runs in seven innings. Each team's half of the inning ends when three of its batters are out. If the score is tied after seven innings, the teams play alternate innings until one team emerges victorious.

The game is played on a dirt infield and grass outfield called a diamond. Bases lie 60ft apart and the imaginary line running between each forms the diamond shaped infield.

The pitcher stands in the middle of the infield on a rubber "plate", pitching (or throwing) the ball to the batter 40ft away. The distance from the home plate to the outfield fence must be 200ft for the Olympic Games, at which fast-pitch softball is played.

A run is scored when a batter, running anti-clockwise, reaches base safely, advances around all three bases and returns to the "home plate" during an inning. The batted ball must land between fixed foul lines before a fielder can catch it in her softball glove. The batter must touch first base before a fielder touches that base with the ball in her hand.

A successful hit and run is not the only way the batter can reach first base. She can do so by batting four balls outside the strike zone - an achievement known as "drawing a walk". The batter may also advance to first base if she is hit on the body by a pitch.

Most points are scored step by step with batters advancing in spurts from base to base but when the batter knocks the ball far enough, usually beyond the outfield, all baserunners and the batter may get round all bases for what is known as a "home run".

If the batter hits a groundball, she must advance to first base, forcing all baserunners before her to move on to the next base, or home. This puts the batting team in its most vulnerable position as fielders need only stand at a base with the ball to ensure that at least one baserunner, if not the batter heading for first base, is called out.

The pitcher must attempt to get the batter out. Forcing a groundball is one way, forcing a high hit for a catch is another, while three strikes (missed balls as far as the batter is concerned) on the batter results in a strikeout.