can send its current to ground through another bulb (at the loss of some brightness), but when the other bulb burns out, they both quit lighting and sometimes a bulb can ground through a filament which is not in use, but can't when that other filament turns on -- ask any old codger like me who's fought with ground problems in '65 Chevy taillights in his youth -- remember the ones on which, when you turn on the turn signals one way, the _taillights_ on the opposite side of the trunk lid would start blinking on and off?