covering 3rd and 1st

Question
Sorry in advance about the length of this question...

3rd baseman straddling the bag while a runner is on 3rd.  Left foot is in
bounds, right foot also has to be in bounds until the start of play - otherwise
it's a balk (I have been told).  What constitutes "start of play"?  Once the
pitcher starts his motion, can the 3rd baseman adjust his right foot and place
it out of bounds - to truly "straddle" the bag?  If he cannot, doesn't his right
foot on the foul line impede the baserunner returning to the bag?  Seems to
be a conflict here.  Thanks for your help!

Answer
Doug,

OBR 4.03 states: "When the ball is put in play ... all fielders other than the catcher shall be on fair territory." OBR 4.02 states the ball is put into play after the fielders and batter have taken their position, and the plate umpire calls "Play."

To answer your question, the balk would be called when the pitcher pitches the ball or attempts a pick-off to a base. If a fielder (other than the catcher) is in foul territory at the time of the pitch (or the pick-off), the umpire should call a balk at that point.

If you have any other questions, or want the answer using another rule code, please let me know.