Normally the beginning of the coil or black wire is grounded to the guitar circuit. On Jaguar pickups the beginning wire is soldered to the metal claw that surrounds the pickup. The beginning wire on a Telecaster bridge pickup also has the beginning black wire connected to the ferrous elevator plate which in turn grounds all the strings and bridge plate by means of the ferrous height adjust screws. If the ferrous elevator plate is removed, the bridge unit and strings are not grounded and can cause the instrument to have increased buzz from electrical interference.

I’ve seen Jazz Basses where the ground wire was connected to the finish eyelet or the outer turns in the coil. Having the hot beginning of the coil in contact with the pole pieces will cause increased noise in the pickup if accidentally touched. Jazz Bass pickups often put and layer of 1/2″ masking tape to insulate the coil from the magnets.

In Mustang guitars, the electrical polarity can be reversed by the slide switches and solid top covers prevent accidental touching of the exposed pole pieces.