Scenario: a ground ball is hit towards third base and bounces left towards foul territory.
Suppose the third baseman grabs the ball while he is standing in fair territory before the ball gets to 3rd base and before the ball touches the ground in foul territory.
Is it a fair ball?
This is a fair ball, the ball never touches foul territory, so therefore it can't be a foul ball.
A FOUL BALL is a batted ball that settles on foul territory between home and first base, or between home and third base, or that bounds past first or third base on or over foul territory, or that first falls on foul territory beyond first or third base, or that, while on or over foul territory, touches the person of an umpire or player, or any object foreign to the natural ground.
A foul fly shall be judged according to the relative position of the ball and the foul line, including the foul pole, and not as to whether the infielder is on foul or fair territory at the time he touches the ball.
(Foul Ball) Comment: A batted ball not touched by a fielder, which hits the pitcher’s rubber and rebounds into foul territory, between home and first, or between home and third base is a foul ball. 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. If a fly ball lands on or beyond first or third base and then bounces to foul territory, it is a fair hit. (Fair Ball (Comment))
Adding to @New-To-IT's great answer, you need to note that whether a ball is foul or not entirely depends on the position of a ball. For example,
If a ball is completely out of the foul line, it is a foul ball whether it is caught or not. Before it crosses the line and is caught by an infielder, it is not a foul ball.
If a ball crosses it and comes back inside the foul line into fair territory, it is not a foul ball.
Therefore, it doesn't matter where a player stands when catching a ball. The only thing that matters is where the ball is.
Wrong, a ball becomes a foul ball immediately upon hitting a fence, backstop, net, umpire, or player while over foul territory before 1st or 3rd base. It matters where the ball is when it is touched until it passes 1st or 3rd base.
