Page "Softball" Paragraph 18

A ball which is hit in the air and caught before hitting the ground is an immediate out, regardless of whether the ball would have landed fair or foul.

A fly ball is a ball hit high and deep, a pop fly is a ball hit high but short, and a line drive is a ball hit close to the horizontal.

In any such situation, runners must remain on their bases until the ball is touched by a defensive player or hits the ground.

If a runner leaves the base before a fly ball, pop fly, or line drive is touched or contacts the ground, the defense can throw the ball to that base, and if the base is tagged before the runner returns, the runner is out as well, resulting in a double play.

If the runner remains on the base until the ball is touched, or returns to the base after the catch but before the defense can put him out, he is said to tag up and may attempt to advance to the next base at his own risk.

If there are less than two batters out and runners on 1st and 2nd bases and the batter hits a pop fly in the infield, the batter is automatically out to prevent unfair play by the fielders.

Unfair play may result from infielders deliberately dropping the ball to try and achieve a double play.

This rule is called the infield fly rule.