Is it legal for the first baseman to have his feet, one or both, in foul territory while holding a runner at first base? Kevin

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Rule you must have eight players in fair territory- catcher is only one that can start in foul territory. The rule is really designed to keep a coach from placing a player behind the catcher during an intentional walk, or some other defensive scheme before a live pitch.

To answer your question directly if you have 1 foot in foul territory and be considered okay, but he may not have both feet in foul territory which by the way is very difficult to do but that would be illegal.

I can tell you right now it's an extraordinarily rare call on first baseman while he is holding a runner on base.  As the proper position for a first baseman right or left-handed is to place his heel on the front inside corner so as to be able to receive the ball, swipe tag, and not have the runner interfere with his receiving the ball by getting in between him and the ball. If your first base with his standing in foul territory he's cheating himself.

Umpire Arnald Swift