A pop fly is hit along the first base foul line between home plate and first base. It lands in fair territory, but with the spin, it bounces three feet into foul territory. Before the ball is allowed to bounce a second time in foul territory, the pitcher catches it in mid-air. No one touched the ball until it was three feet in foul territory, when the ball was caught in mid-air. Is this a fair or a foul ball, since it took place inside the first base bag?
The definition for "Fair Ball" in the MLB rules say:
If a fly ball lands in the infield between home and first base, or home and third base, and then bounces to foul territory without touching a player or umpire and before passing first or third base, it is a foul ball; or if the ball settles on foul territory or is touched by a player on foul territory, it is a foul ball.
Therefore, this scenario is a foul ball, given the pitcher who catches it in mid-air is also in foul territory.
It is a foul ball since it went foul before it passed first base. It does not matter that it did not touch the ground in foul territory. It is determined to be foul based on where the ball is when it is touched by the fielder.
Note: it does not matter where the fielder is standing when they touch it, only where the ball is in relation to the foul line. This is the umpire's judgment.