Pickups typically come in two main varieties: Single coils and humbuckers, although hybrids and other possibilities are presenting themselves in the market all the time, and are increasingly becoming mainstream components.

Single coil pickups
Humbucking pickups
    These pickups derive their name from the fact that they eliminate or "buck" the 60 cycle hum that plagues standard single-coil pickups.  They do this by wiring two coils such that the hum of one coil is canceled by a "mirror image" of the hum from the other coil.  These pickups are also, with few exceptions, wired in series (whereas single coil pickups are usually combined in parallel), such that the sound produced tends to be warm and thick.  However, a tremendous amount of variety exists in the market, so almost any color of pickup is available if one is willing to look.  Another distinction between humbuckers and single coils is that one or both coils of a humbucker typically have adjustable pole pieces.  These can be adjusted with a screwdriver (or Allen wrench, as is the case with DiMarzios) to conform to the string height.  Of course, there are dozens of variations in appearance, including the minimalist look of EMG pickups or the aforementioned "stacked" humbuckers that look (and often sound) in every other way like single coils models.  Variety is the spice of life!

Piezo pickups
    These pickups operate on a whole other principle than the magnetic variety above.  Rather than the string generating electricity by vibrating through a magnetic field, these pickups transduce physical vibrations into electricity.  This is accomplished by piezo-electric materials such as quartz.  This is exactly how record player needles and heart monitors operate as well.  Typically, these pickups are found in acoustic-electric guitars of all makes and models (although the higher-end varieties frequently have an internal microphone as well).  Also, many innovative guitar makers are incorporating these into their models, including names such as Parker, Godin, and even Ibanez.  Fishman also makes an after-market Strat style tremolo with incorporated piezo string saddles.

Piezo transducers

If you notice any parts I left off this little primer, please email me.

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