Play:In MLB if a defensive player other than the catcher is in foul territory at the time of the pitch, is it a balk if runner(s) are on base?
Ruling: At the MLB level as well as the high school level, and any other level for that matter, the catcher is the only defensive player who is permitted by rule to be standing in foul territory at the time of the pitch. If any other player is positioned in foul territory, such as by placing a foot completely outside the 1st or 3rd base line, at the time of the pitch, the home plate umpire may rule an illegal pitch. With a runner or runners on base, any pitch that is made is a balk.
Sources: MLB Rule 4.03(a); NFHS 1.1.4 and 2.18