Before the Pitch

Pitcher is ready to throw and batter is ready to hit. The batter's feet must be entirely within the batter's box.  The lines are considered to be completely within the box.

If the umpire sees this condition, call time and request the batter getter properly set in the box.

At the Time of Bat Contact

When the bat makes contact (fair or foul) with the ball, some portion of both feet must be within the box.  Again, the lines of the batter's box are considered to be wholly inside the box.

If illegal contact is made, then the batter is out and the ball is dead, all runners being returned to their base of origin at the time of the pitch.