Pickups are electronic devices attached to a guitar that detect (or "pick up") string vibrations and allow the sound of the string to be amplified. Pickups are usually placed right underneath the guitar strings. The most common type of pickups contain magnets that are tightly wrapped in copper wire. Pickups work on a similar principle to a electrical generator in that the vibration of the strings causes a small voltage to be created in the coils surrounding the magnets. This signal is later amplified by an amplifier.
Traditional electric pickups are either single-coil or double-coil. Double-coil pickups are also known as humbuckers for their noise-cancelling ability. The type and model of pickups used can greatly affect the tone of the guitar. Typically, humbuckers, which are two magnets attached to each other and each wrapped in copper wire, are used by guitarists seeking a heavier sound. Single coil pickups, one magnet wrapped in copper wire, are used by guitarists seeking a brighter, twangier sound with greater dynamic range. However, a disadvantage of single coil pickups is mains-frequency (60 or 50 hertz) hum. Some guitars need a battery to power their pickups and/or pre-amp; these guitars are referred to as having "active electronics", as opposed to the typical "passive" circuits.
The Fender Stratocaster type guitars generally utilize 3 single coil pickups, while the Gibson Les Paul types use humbucker pickups.
A further type of pickup is the piezo pickup. These employ piezoelectricity to generate the musical signal and are popular in hybrid electro-acoustic guitars. Usually, a crystal is located in the saddle under each string. When the string vibrates, the shape of the crystal is distorted, and this change in shape produces a tiny voltage that can be amplified and manipulated.
Some guitars have what is called a hexaphonic pickup. These pickups are also piezo pickups. "Hex" is a prefix meaning six. In a hexaphonic pickup there is a separate piezo pickup wired for each of six strings. This arrangement allows the signal to be easily modified by on-board modelling electronics, as in the Line 6 Variax brand of electric guitars, the guitars allow for a variety of different sounds to be obtained by digitally modelling the vibration. This results in a guitar which is able to mimic many vintage models, as well as output alternate tunings (e.g. Drop D) without the need to adjust the strings. The benefits of using a piezo pickup include the ability to bend strings and use palm/neck muting. Another use for hexaphonic pickups is to send the signal (that is the six separate signals) to a MIDI interpretation device, which determines the note pitch, duration, attack and decay characteristics, and so forth. The MIDI (Musical Instrument Digital Interface) interpreter then sends the note information to a sound bank device. The resulting sounds can closely mimic a piano, trumpet or other brass instrument, harmonica or any of numerous other instruments.