A bobbin can be made in various shapes depending on the desired number of strings to be used. Bobbins can be tall and have narrow coils like Fender Stratocasters or have short and wide coils like a Fender Jazzmaster. Fender Strat bobbins have a .434″ traverse and a Jazzmaster is close to .125″ traverse. Gibson humbuckers have an average traverse of .245″ and but may have specified .250″ but the molded plastics can shrink a little. Bobbins are normally used to hold the magnet wire that’s wound with a specific number of turns and gauge of magnet wire.