What are -12v and -5v supplies?

[haaammit]

Hey all. I'm in the process of finding a permanent 5v power supply from my psu and with various research have found that pin 9 on the 24 pin atx connector is the one I want.

Looking at the various pinouts I've seen that pin 14 is -12v (designated blue wire) and pin 20 is/was -5v (designated white cable, often not connected). Neither of these supplies are used much in systems these days, but how does a -12v or -5v work? I thought maybe the polarity would be different, but that doesn't work with common grounds, does it?

I mean it's obviously not going to be sucking 12 or 5 volts back up the wires, but what's going on?

Sorry if this is a dumb question, but I can't seem to find an answer! Many thanks

 

[haaammit]

Electricity confuses me sometimes with stuff like this lol, but thanks for the replies. I'm still not 100% sure on how the - supply works, but it was just out of curiosity!

The item I want to power is a remote reciever that plugs into an always powered usb header, so is designed to only need a very low current as supplied by the 5VSB supply here All my motherboard header cables are custom made and I cba to make another so was going to pinch off the 5vsb supply directly out of the psu and directly wire the adaptor up.