good question. i have no clue
There are lines placed to divide the foul territory and the fair territory. Any ball that lands on the foul territory will be called foul. When the ball enters the fair territory, ball is at play.
Once the ball is past first or third base, the ONLY consideration is where the BALL is located -- fair or foul -- when it FIRST comes in contact with either the ground or a player. If the player is almost entirely in fair territory when he first grabs the ball, but the glove that touches the ball is in foul territory, then it's a foul ball. "If the ball touches a fielder in-flight, the judgment is made at where the ball was when it was touched, NOT from where it may land after a miss, or drop of the ball, by a fielder. The position of the fielder is irrelevant."