Position A for the base umpire is defined as being in foul territory, squared up to home plate, and in a set position. A set position will be with the feet slightly beyond the shoulders, with the arms locked, but not rigid, with the thumbs on the inside of the knees. From this position...
He will watch the pitcher. He will also be responsible to be available on a check swing to help his partner, to watch for any balls that hit the batter, whether it be a pitch ball or a batted ball, and also for anything that would possibly go up and out of play. Position A will be determined by the positioning of the first baseman.
If the first baseman is in normal position, you will be approximately 10 to 20 yards away from the first baseman. 12 feet behind the first baseman to give him an opportunity to make a play on the ball. If he moves in and plays in front of the bag, you will never go any closer than 10 to 12 feet to first base. In the A position, the base umpire also has the fair foul responsibility from the front edge of first base all the way through to the foul pole in right field.
This is the only time that the base umpire will have a fair foul. file responsibility.