Pickups are supplied in either Single Conductor or 4 Conductor Wire versions. Single Conductor The single conductor pickup has a wire leading out from the pickup which consists of an outer braiding with a single inner coated conductor. In Seymour Duncan pickups usually white.(Hence single conductor wire )  The outer braiding also acts as a conductor even though it may look like the protection for the inner conductor.  The Outer braiding is the earth or ground  for the pickup and must be connected to the guitar ground  usually to the rear of one of the control pots. The inner conductor (White) is the live or hot output from the pickup 4 Conductor Wire The 4 conductor wire pickup is a little confusing as it actually has 5 wires leading out from the pickup the wires are colour coded with the following colours:- Red White Black Green And a bare conductor The Red and White wires are usually soldered together as this is the centre tap of the two pickup coils.  This provision, if required, may be used with a separate switch to provide coil tapping. See wiring diagrams at:- www.seymourduncan.com  When coil tapping is not required the soldered together Red and White wires are simply taped up and positioned aside (not used.) Black is the hot or live output from the pickup usually connected to the volume control or the guitar 3 or 5 way  switch. The Green is the common Ground connection for the pickup coils and is usually solider together with the bare wire to the guitar ground (see below)   The bare conductor is the ground connection for the pickups mounting plate and is usually soldered to the green conductor (above) and both are normally connected to the guitar ground.