Pickups

Compared to an acoustic guitar, which has a hollow body, electric guitars make comparatively little audible sound simply by having their strings plucked, and so electric guitars are normally plugged into a guitar amplifier, which makes the sound louder. When an electric guitar is strummed, the movement of the strings generates (i.e., "induces") a very small electrical current in the magnetic pickups, which are magnets wrapped with coils of very fine wire. That current is then sent through a cable to a guitar amplifier. The current induced is proportional to such factors as the density of the string or the amount of movement over these pickups. That vibration is, in turn, affected by several factors, such as the composition and shape of the body.

A close-up of the pickups on a Fender Squier "Fat Strat" guitar; on the left is a "humbucker" pickup and on the right are two single-coil pickups.
Some "hybrid" electric-acoustic guitars are equipped with additional microphones or piezoelectric pickups (transducers) that sense mechanical vibration from the body. Because in some cases it is desirable to isolate the pickups from the vibrations of the strings, a guitar's magnetic pickups will sometimes be embedded or "potted" in epoxy or wax to prevent the pickup from having a microphonic effect.
Because of their natural inductive qualities, all magnetic pickups tend to pick up ambient and usually unwanted electromagnetic noises. The resulting noise, the so-called "hum", is particularly strong with single-coil pickups, and aggravated by the fact that very few guitars are correctly shielded against electromagnetic interference. The most frequent cause is the strong 50 or 60 Hz component that is inherent in the generation of electricity in the local power transmission system. As nearly all amplifiers and audio equipment associated with electrical guitars rely on this power, there is in theory little chance of completely eliminating the introduction of unwanted hum.
Double-coil or "humbucker" pickups were invented as a way to reduce or counter the unwanted ambient hum sounds (known as 60 cycle hum). Humbuckers have two coils of opposite magnetic and electric polarity. This means that electromagnetic noise hitting both coils should cancel itself out. The two coils are wired in phase, so the signal picked up by each coil is added together. This high combined inductance of the two coils leads to the richer, "fatter" tone associated with humbucking pickups. Optical pickups are a type of pickup which sense string and body vibrations using infrared LED light.

Tremolo arms
Some electric guitars have a tremolo arm (sometimes called a "whammy bar" or "vibrato arm" and occasionally abbreviated as trem), a lever attached to the bridge which can slacken or tighten the strings temporarily, changing the pitch, thereby creating a vibrato or a portamento effect. Early vibrato systems, such as the Bigsby vibrato tailpiece, tended to be unreliable and cause the guitar to go out of tune quite easily, and also had a limited range. Later Fender designs were better, but Fender held the patent on these, so other companies used Bigsby-style vibrato for many years.

Detail of a Squier-made Fender Stratocaster. Note the vibrato arm, the 3 single-coil pickups, the volume and tone knobs.
With the expiration of the Fender patent on the Stratocaster-style vibrato, various improvements on this type of internal, multi-spring vibrato system are now available. Floyd Rose introduced one of the first improvements on the vibrato system in many years when in the late 1970s he began to experiment with "locking" nuts and bridges which work to prevent the guitar from losing tuning even under the most heavy whammy bar acrobatics.

Guitar necks
Electric guitar necks can vary according to composition as well as shape. The primary metric used to describe a guitar neck is the scale, which is the overall length of the strings from the nut to the bridge. A typical Fender guitar uses a 25.5 inch scale, while Gibson uses a 24.75 inch scale in their Les Paul. While Gibson's scale has often claimed to be 24.75", it has varied through the years by as much as a half inch. The frets are placed proportionally according to the scale length; thus, the smaller the scale, the tighter the spacing of the frets.
Necks are described as bolt-on, set-in, or neck-through depending on how they are attached to the body. Set-in necks are glued to the body in the factory, and are said to have a warmer tone and greater sustain; this is the most traditional type of joint. Bolt-on necks were pioneered by Leo Fender to facilitate easy adjustment and replacement of the guitar neck. Neck-through instruments extend the neck itself to form the center of the guitar body, and are known for long sustain and for being particularly sturdy. While a set neck can be carefully unglued by a skilled luthier, and a bolt-on neck can simply be unscrewed, a neck-through design is difficult or even impossible to repair, depending on the damage. Historically, the bolt-on style has been more popular for ease of installation and adjustment; since bolt-on necks can be easily removed, there is an after-market in replacement bolt-on necks from companies such as Warmoth and Mighty Mite. Some instruments, notably most Gibson models, have continued to use set/glued necks. Neck-through bodies are somewhat more common in bass guitars.

The materials used in the manufacture of the neck have great influence over the tone of the instrument. Hardwoods are very much preferred, with maple, ash, and mahogany topping the list. The neck and fingerboard can be made from different materials, such as a maple neck with a rosewood fingerboard. In the 1980s, exotic man-made materials such as graphite began to be used, but are pricey and never have replaced wood in production instruments. Such necks can be retrofitted to existing bolt-on instruments.
There are several different neck shapes used on guitars, including shapes known as C necks, U necks, and V necks. These refer to the cross-sectional shape of the neck (especially near the nut). There are also several sizes of fret wire available, with traditional players often preferring thin frets, and metal shredders liking thick frets. Thin frets are considered better for playing chords, while thick frets allow lead guitarists to bend notes with less effort. An electric guitar with a neck which folds back called the "Foldaxe" was designed and built for Chet Atkins by Roger C. Field (featured in Atkins' book "Me and My Guitars."). Steinberger guitars developed a line of exotic instruments without headstocks, with tuning done on the bridge instead.

Sound and effects
While an acoustic guitar's sound is largely dependent on the vibration of the guitar's body and the air within it, the sound of an electric guitar is largely dependent on a magnetically induced electrical signal, generated by the vibration of metal strings near sensitive pickups. The signal is then "shaped" on its path to the amplifier by using a range of effect devices or circuits that modify the tone and characteristics of the signal. The amplifiers and speakers used also add (intentional) coloration to the final sound.

Built-in sound shaping
Electric guitars can have up to three magnetic pickups. Identically positioned pickups will have different tones depending on how near they are to the neck or bridge, with bridge pickups having a bright or trebley timbre, and neck pickups being more warm or bassey. The type of pickup also effects tone, with dual-coil pickups sounding warmer, thicker, perhaps even muddy, and single coil pickups sounding clear, bright, perhaps even biting. Guitars do not have to be fitted with a uniform type of pickup: a common mixture is the "fat strat" arrangement of one dual-coil at the bridge position, with single coils in the middle and neck positions.
Where there is more than one pickup, selector switching is fitted. These often allow the outputs of two or more pickups to be combined, so that two-pickup guitars have three-way switches, and three-pickup guitars have five-way switches. Futher circuitry is sometimes provided to combine the pickups in different ways. For instance, phase switching places one pickup out of phase with the other(s), leading to a "honky", "nasal", or "funky" sound. Individual pickups can also have their timbre altered by switches, typically coil tap switch, which effectively short-circuits some of a dual-coil pickup's windings, giving a tone like a single coil pickup.
The final stages of on-board sound-shaping circuitry are the volume control (potentiometer) and tone control (which "rolls off" the treble frequencies). Where there are individual volume controls for different pickups, and where pickup signals can be combined, they would affect the timbre of the final sound by adjusting the balance between pickups from a straight 50:50.
The strings fitted to the guitar also have an influence on tone. Rock musicians often prefer the lightest gauge of roundwound string, which are easier to bend, while jazz musicians go for heavier, flatwound strings with a rich, dark sound.
Recent guitar designs may incoporate much more complex circuitry than described above: see Digital and synthesizer guitars, below.

Classic amplifier sounds
In the 1960s, some guitarists began exploring a wider range of tonal effects by distorting the sound of the instrument. To do this, they used overdrive — increasing the gain, of the preamplifier beyond the level at which the signal could be faithfully reproduced, resulting in a "fuzzy" sound. This effect is called "clipping" by sound engineers, because when viewed with an oscilloscope, the wave forms of a distorted signal appear to have had their peaks "clipped off", approximating a square wave. This was not actually a new development in the instrument, but rather a shift of aesthetics, the sound having not been recognized as desirable previously.
Distortion achieved by overdrive necessarily involves high volumes and is therefore often combined with audio feedback.
After distortion became popular, amplifier manufacturers included various provisions for it, making amps easier to overdrive, and providing separate "dirty" and "clean" channels so that distortion could easily be switched in and out. The distortion characteristics of vaccuum tube amplifiers are particularly sought-after, and various attempts have been made to emulate them without the disadvantages (fragility, low power, expense) of actual tubes.
Guitar amplifiers have long included at least a few effects, often tone controls and a spring reverb unit. The use of offboard effects is assisted by the provision of effect loops, an arrangement that allows effects to be taken out of circuit when not required.

Effects units

In the 1960s, the tonal palette of the electric guitar was further modified by introducing an Effects unit in its signal path. modifiers, wave-shaping circuits, voltage-controlled oscillators, or digital delays. Effects units come in several formats, the most common of which are the stomp-box and the rack-mount unit. A "stomp box" (or "pedal") is a small metal or plastic box containing the circuitry which is placed on the floor in front of the musician and connected in line with the patch cord connected to the instrument. The box is typically controlled by one or more foot-pedal on-off switches and it typically contains only one or two effects. "Guitar pedalboards" are used by musicians who use multiple stomp-boxes; these may be a DIY project made with plywood or a commercial pedalboard.
A rack-mount effects unit may contain the identical electronic circuit, but is mounted in a standard 19" equipment rack. Usually, however, rack-mount effects units contain several different types of effects. They are typically controlled by knobs or switches on the front panel, and often by a MIDI digital control interface.
Typical effects include:-
•    Effects such as stereo chorus, phasers and flangers which shift the pitch of the signal by a small and varying amount, creating swirling, shimmering and whooshing noises.
•    Effects such as octavers, which displace pitch by an exact musical interval.
•    Distortion, such as transistor-style fuzz, or effects incorporating or emulating vaccuum tube distortion.
•    Filters such as wah-wah
•    Envelope shapers, such as compression/sustain or volume/swell.
•    Time-shift effects such as delay and reverb.
Not all special effects are electronic; in 1967, guitarist Jimmy Page of The Yardbirds and Led Zeppelin created unusual, psychedelic sound effects by playing the electric guitar using a violin bow with a delay pedal and wah pedal.

Modern amplifier techniques
In the 1970s, as effects pedals proliferated, their sounds were combined with tube amp distortion at lower, more controlled volumes by using power attenuators such as Tom Scholz' Power Soak as well as re-amplified dummy loads such as Eddie Van Halen's use of a variac, power resistor, post-power-tube effects, and a final solid-state amp driving the guitar speakers. A variac is one approach to power-supply based power attenuation, to make the sound of power-tube distortion more practically available.
Recent amplifiers may include digital technology similar to modern effects pedals, including the ability to model or emulate a variety of classic amps.

Digital and software-based effects

A multi-effects device (also called a "multi-FX" device) is a single electronics effects pedal or rackmount device that contains many different electronic effects. In the late 1990s and throughout the 2000s, multi-FX manufacturers such as Zoom and Korg produced devices that were increasingly feature-laden. Multi-FX devices allow several of the effects to be used together, and most devices allow users to set "preset" combinations of different effects including distortion, chorus, reverb, compression, and so on. This allows musicians to have quick on-stage access to different effects combinations. Some multi-FX pedals for contain modelled versions of well-known effects pedals or amplifiers.

The Boss GT-8 is a higher-end multi-effect processing pedal; note the preset switches and patch bank footswitches and built-in expression pedal.
Multi-effects devices have garnered a large share of the effects device market because they offer the user such a large variety of effects in a single package. A low-priced multi-effects pedal may provide 20 or more effects for the price of a regular single-effect pedal. More expensive multi-effect pedals may include 40 or more effects, amplifier modelling, and the ability to combine effects and/or modelled amp sounds in different combinations, as if the user was using multiple guitar amps. More expensive multi-effects pedals may also include more input and output jacks (e.g., an auxiliary input or a "dry" output), MIDI inputs and outputs, and an expression pedal, which can control volume or modify effect parameters (e.g., the rate of the simulated rotary speaker effect).
By the 1980s and 1990s, software effects became capable of replicating the analog effects used in the past. These new digital effects attempted to model the sound produced by analog effects and tube amps, to varying degrees of quality. There are many free guitar effects computer programs for computers that can be downloaded via the Internet. Now, computers with sound cards can be used as digital guitar effects processors. Although digital and software effects offer many advantages, many guitarists still use analog effects.

Synthesizer and digital guitars
In 2002, Gibson announced the first digital guitar, which performs analog-to-digital conversion internally. The resulting digital signal is delivered over a standard Ethernet cable, eliminating cable-induced line noise. The guitar also provides independent signal processing for each individual string. Also, in 2003 amp maker Line 6 released the Variax guitar. It differs in some fundamental ways from conventional solid-body electrics. For example it uses piezoelectric pickups instead of the conventional electromagnetic ones, and has an on-board computer capable of modifying the sound of the guitar to model the sound of many instruments.

 Playing techniques

The sound of a guitar is not only adapted by electronic sound effects, but also heavily by all kinds of new techniques developed or becoming possible in combination with the electric amplification. This is called extended technique.
Extended techniques include:-
•    String bending (or radial finger vibrato.). This is not quite unique to the electric instrument, but is greatly facilitated by the light strings typically used on solid body guitars.
•    The use of the whammy bar or "tremolo" arm, including the extreme technique of dive bombing.
•    Tapping, in which both hands are applied to the fretboard. This is only feasiible with the assistance of amplification.
•    Pinch harmonics.
•    Use of audio feedback to enhance sustain and change timbre.
•    Substitution of another device for the plectrum, for instance the violin bow (as famously used by Jimmy Page) and the e-bow, (a device using electromagnetic feedback to vibrate strings without direct contact). Like feedback, these techniques increase sustain, bring out harmonics and change the acoustic envelope.
•    Sustainers built into the guitar itself
•    Use of slide or bottlenecks.
•    Sometimes guitars are even adapted with extra modifications to alter the sound, such as Prepared guitar and 3rd bridge.
Other techniques such as axial finger vibrato, pull-offs, hammer-ons, palm muting, harmonics and altered tunings are also used on the classical and acoustic guitar. Shred guitar is a genre involving a number of extended techniques.