One thing to confuse things, but interesting, is when working on high voltage things like a regulator, or an rf generating device that is operating at high voltages. They often have the filaments at a potential of say 5-7 volts dc, while the stuff around them is at 5000vdc or higher. If you measured from the filament to ground, you'd get maybe 5005, but if you measured between the two leads, 5 volts. This is where understanding what the reference, or bonded point, can be very interesting and dangerous. Neither side of the filaments in this case are referenced or bonded to ground, but one side of the filaments is bonded to one side of the high voltage supply. They do this so you don't end up with arcing internal to the tube.