ASA Rule 7-3-A: Prior to the pitch, the batter must have both feet completely within the lines of the batter's box. The batter may touch the lines, but no part of the foot may be outside the lines prior to the pitch.
I saw this with two batters last night, each time they came to bat. Half of both feet were outside the box. This is my weekly night to read the rule book, and I just read the rule listed above. I don't see an effect listed for this. So, here's my call: I request the batter to step completely in the box. I will assume the batter cooperates and play continues.
What if the next time the batter comes to the plate he assumes his original stance?
I know and understand 7-6-D & E, concerning the batter's foot outside the box (or touching home plate) at the time the ball makes contact, but that is a different sitch than this.