Electric guitars can have solid, semi-hollow, or hollow bodies, and produce little sound without amplification. Electromagnetic pickups (single and double coil) convert the vibration of the steel strings into electrical signals which are fed to an amplifier through a cable or radio transmitter. The sound is frequently modified by other electronic devices or the natural distortion of valves (vacuum tubes) in the amplifier. The electric guitar is used extensively in jazz, blues and rock and roll, and was commercialized by Gibson together with Les Paul and independently by Leo Fender of Fender Music. The lower fretboard action (the height of the strings from the fingerboard) and its electrical amplification lend the electric guitar to some techniques which are less frequently used on acoustic guitars. These techniques include tapping, extensive use of legato through pull-offs and hammer-ons (also known as slurs in the traditional Classical genre), pinch harmonics, volume swells and use of a Tremolo arm or effects pedals. Seven-string solid body electric guitars were developed in the 1980s. Throughout the late 80's and 90's the seven string was popularized by the creation of the Ibanez Jem. The Jem was developed by Ibanez with close specifications and a specific feel that Steve Vai helped develop and master. Vai popularized the seven string and the seven string is heard in much of the rock music these days (earlier in jazz) to achieve a much darker sound through extending the lower end of the guitar's range. They are used today by players such as James "Munky" Shaffer, Dave Weiner, John Petrucci, Jeff Loomis, Steve Smyth, and Steve Vai. Meshuggah, Dino Cazares, Rusty Cooley & Charlie Hunter go a step further, using an 8 string guitar with two extra low strings. Although the most commonly found 7 string is the variety in which there is one low B string, Roger McGuinn (Of Byrds/Rickenbacker Fame) has popularized a variety in which an octave G string is paired with the regular G string as on a 12 string guitar, allowing him to incorporate chiming 12 string elements in standard 6 string playing. Ibanez makes many varieties of electric 7 strings.
However, the most common method of achieving a darker, deeper sound is to tune the 6th string (E) to a low D, known as a 'drop D tuning'. Many of today's 'Dark Metal' and 'Nu Metal' bands use this tuning to add extra heaviness to their sound. Devin Townsend uses an 'open G' tuning to achieve his particular heavy sound. Edward Van Halen sometimes uses a device known as a 'Drop D Tuner' which is a small lever attached to the tuner of the 6th string which allows him to easily drop that string's tuning to a D.
The electric bass guitar is similar in tuning to the traditional double bass viol. Hybrids of acoustic and electric guitars are also common. There are also more exotic varieties, such as double-necked guitars, all manner of alternate string arrangements, fretless fingerboards (used almost exclusively on bass guitars, meant to emulate the sound of a stand-up bass), 5.1 surround guitar, and such.
Some electric guitar and electric bass guitar models feature Piezoelectric pickups, which function as small microphones to provide a sound closer to that of an acoustic guitar with the flip of a switch or knob, rather than switching guitars.