This usually refers to a wire brought out from the connection between the two coils of a humbucker. The 4-wire versions bring all the wires out where you can hook them up any way you want to.

By connecting the two coils in series, you get the hot, 'chunky' sound, that is typical of humbuckers.

Parallel connection provides a lower output voltage, and the sound usually has a bit more clarity.

Both of these connections retain the hum rejection feature of the humbucker.

One of the coils can be shorted out, leaving the other one active. This produces something closer to the sound of a Strat or Tele type pickup, but loses hum rejection.

[Edited by Lordathestrings on 10-18-2001 at 10:09 PM]