A regulation baseball field has two batter's boxes -- one on the left side and one on the right side of home plate -- drawn using the same chalk as the baselines. From the pitcher's point of view, left-handed batters stand in the batter's box on the left side of the plate and right-handed batters stand in the batter's box on the right side of the plate.
The batter is expected to promptly take his place in the batter's box when his turn to bat comes up and is not permitted to exit the box once the pitcher begins his windup or comes to the set position. The batter can request to be granted time by the umpire if the pitcher has not begun his windup or come to the set position. If his request is granted, the batter is then able to exit the box -- but not the dirt area surrounding home plate. He may also exit the box in other select instances, such as after a swing or bunt attempt, a passed ball, a wild pitch or an inside pitch that forces him out of the box. Furthermore, he may exit the box if the pitcher or catcher vacates his respective position, a member of the defensive team requests time or attempts a play on a runner at any base, or an attempted check swing is appealed to a base umpire. Otherwise, the batter is expected to keep at least one foot inside the box throughout his time at bat.
If a batted ball that has not yet been touched by a fielder makes contact with a batter who still has two feet in the batter's box, the ball will be ruled foul. But if the batter has already exited the batter's box before being contacted by a fair ball that has not yet been touched by a fielder, the batter will be called out.
Watch: Adrian Beltre steps out of the batter's box, realizes it and quickly steps back into the box.