In baseball, a fair ball is a batted ball that has not yet become a foul ball, and that...
settles on fair ground between home and first base, or between home and third base, or
is on or over fair territory when bounding to the outfield past first or third base, or
touches first, second, or third base, or
touches the person of an umpire or player while the ball is on or over fair ground, or
passes out of the playing field in flight while over fair ground.
It is possible for a ball moving in foul territory to become a fair ball. Batted balls can also be foul balls or foul tips.
On a fair ball, the ball is alive; runners may attempt to advance and fielders may try to record outs. A batted ball is presumed to be fair until it is ruled a foul ball or a foul tip.
NOTE: A fair fly shall be adjudged according to the relative position of the ball and the foul line, including the foul pole, and not as to whether the fielder is on fair or foul territory at the time such fielder touches the ball.
A ball is in fairterritory if any part of it is on or over any part of the foul line.
The third baseman is careful to keep his feet in fairterritory he fields a ball in front of the base.
Fairterritory and foul territory in baseball or softball.
It is either amusing or confusing that the lines are officially called the foul lines and the poles are called the foul poles even though they are all in fairterritory, as is the wall that stretches between the poles.
The shape of foul territory is different in each baseball park; it depends on how close the stands are to the playing field.