Originally Posted by
Pilot DAR
The certification requirement for GA airplanes states that for there to be a "both" fuel selector position, it is required to interconnect the vent spaces of the left and right tanks with a vent tube with no low spot. The geometry of a low wing aircraft with dihedral makes it impossible to do this, so only "L" and "R" positions are permitted by design arrangement.
It's not impossible to do in a low wing aircraft. I know of at least one low wing aircraft which operates with the selector effectively set to both (It only has an ON/OFF fuel valve) and has an interconnected vent tube.