At this point we say that the Wheatstone bridge is balanced. Now let’s say we don’t know the resistance of R4 (let’s instead call it Rx). And let’s connect A and C through a galvanometer ...

This messes up the stereo balance, so naturally it’s worth selecting a part with good matching. [Keri] selects them not with his golden ears, but by wiring both pots together as a Wheatstone bridge.