Below is an alphabetical list of the most commonly used guitar effects with an explanation of how each one sounds or works.

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A 
Automatic Double-Tracking - also known as ADT, a delay-based effect. An effect created using either a digital or analogue unit capable of producing a very fast single repeat of the input signal. This creates an effect often referred to as doubling. 

B
Bypass - the act of switching the instrument signal to a route whereby it is not processed, most effects units/pedals can bypass a signal. 

C
Chorus - another delay based effect in the region of 15 and 35 milliseconds. It simulates what happens when two guitars play the same part. Another type of doubling. But as with real-life double-tracking, there are always discriminations in the pitch and timing between parts - a chorus recreates this effect electronically. Most chorus effect units have controls to vary both the rate (modulation) and depth of the outputted signal. 

Compressor - a signal averaging out effects unit. When using compression, quiet notes are boosted in volume, while louder signals - such as a heavily struck chord - are reduced in volume. This averages out the natural differences in level as you play, making finger-picking sound smoother, and giving a flowing feel to clean lead passages. Wide arrays of effects are achievable with a compressor, by adjusting the threshold and compression controls. For example, heavy compression will reduce the percussive front of a heavily picked note to give it a softer start; as the note fades, he unit increases volume, thus keeping the level constant. 

D
Delay - when sound is reflected from a distant surface, a delayed version of the original signal is heard a short time later. Echo-units electronically recreate this natural effect, by either analogue or digital means. Solid-state units store the signal electronically; analogue devices pass the signal down a long chain until it is needed to produce the echo, while in digital unit?s the signal is stored in an encoded form until it is required, it is then decoded and used. Digital units can produce stereo delays, allowing multi-tapped signals to be played left, right and centre-stage. It is even possible to produce a ?ping-pong? delay, where the sound appears to bounce from side to side. 

Digital effects - almost every effect can be produced digitally. When using digital effects, the signal is converted to a binary code - a series of ones and zeros - so that can be processed and then converted and played back as an analogue signal. 

Distortion - when extra gain and distortion cannot be achieved by an amplifier, traditionally a distortion pedal (a fuzz-box/overdriver) is used. A clean signal is plugged in, and a distorted, sustaining sound is produced. The amount and type of distortion can be controlled on the effects unit. Some units include value based pre-amps to give good tone and dynamics coupled with genuine value overdrive. Digitally produced distortion, while very common, does not usually have the warmth of analogue valve based distortion and is often thought to be too harsh for the tastes of most guitarists. 

E
Enhancers - this type of device processes an audio signal to improve the sound and definition. Early enhancers where known as ?aural exciters? and boosted a harmonic element in the music to produce a brighter effect. Other systems use phase correction to place signals precisely in phase so that frequencies are not lost as a result of phase cancellation. 

Expander - the opposite effect of compression is called expansion. Used to create an increased dynamic range of a signal. 

F
Flanger - a delay-based effect that originated with tape recorders. The tape was slowed down by pressing the fingers against the reel, and the sound produced was mixed together with the normal signal from a second tape. The flanging sound is created electronically by playing back a delayed signal (of up to 20 milliseconds) with a controlled pitch modulation, against the original track. Most modern day effects pedals can produce this effect. 

G
Graphic equalizer - a device which controls the tone of a signal by splitting it into a spectrum of frequency-bands, allowing each band to be boosted or cut separately. The word ?graphic? refers to the fact that it is possible to see at a glance what particular ?shape? is being used. E.g. a V-shape boosts the top end, while the opposite response adds mid-range warmth. Only tonal emphasis can be changed, it is not possible to improve the basic quality of the tone. 

H
Harmonizer - also known as the pitch-shifter. This effect has two main uses. It can enrich the sound of a guitar, using a harmonizer to add overtones which are in harmony with the original signal (sounds similar to chorus). The harmonizer can also generate a harmony note. Until recently, only fixed-interval harmonies were possible; however, modern electronics can intelligently adjust to produce specific user defined harmonies. 

L
Leslie cabinet - the Leslie is a rotary speaker cabinet designed for use with an organ. In the 1960?s, players such as Jimi Hendrix found that by feeding the guitar signal though a Leslie cabinet they sound produce a delicate, ethereal sound. Nowadays the effect is electronically produced. 

M
MIDI - stands for Musical Instrument Digital Interface. Developed in 1981 by the Sequential Circuits Company as a universal interfacing system for synthesizers and sequencers. MIDI is widely used within effects units as a way of controlling parameters or stored settings from either footswitches or sequencers. 

Multi-effects units - a single unit that is capable of producing reverb, flanging, delay, chorus, phasing, harmonising, distortion and many other effects. These devices are usually digital, MIDI-controllable, and capable of chaining effects together and storing settings. 

O
Octave divider - this analogue effect (an early forerunner of the harmonizer) added a single note either an interval of an octave above, or an octave below the original signal. 

P
Panning - the location of a signal within a stereo field. It can also refer to the dynamic behaviour of the signal - for example, where echo repeats are panned from left to right. 

Phasing - if two identical versions of a signal are ?out-of-phase?, so that the peaks in one coincide with the troughs in the other, the two signals will cancel each other out, leading - in theory - to silence. If the signals are partially out-of-phase a characteristic colouration to the sound will result. Phasing can be achieved electronically, the results varying from a mild ?whooshing? to sounds reminiscent of a jet plane. 

Pre-amplifier - to help overload the input-stages of the amplifier, a ?pre-amp? can be used to generate extra gain. The pre-amp often acts as a tone control when used in conjunction with a main amplifier. It can also be used to boost the signal of an acoustic instrument when used with an amplifier. 

R
Reverberation - a reverb unit mimics the natural effect of overlapping sound reflections caused by sound bouncing around an interior space such as a room. Spring reverb is the traditional effect built into many amplifiers, but digital reverb (offering fine control of all the parameters) is now more common. 

S
Stereo chorus - this term can refer to a chorus effect that is panned over stereo outputs to give the impression of spatial movement. It can also describe the sound created by playing a dry signal through one channel and a chorused signal though the other. 

T
Tape-echo - the traditional method of producing delay. The original signal is recorded on a tape and played back slightly later by one or more replay heads, giving either a single repeat or ?multi-tap? effects. Feeding the delayed signal back to the recording heads gives a heavily textured sound. 

Tone pedal - see wah-wah pedal. 

Treble booster - in the early 1960?s, many of the cheaper amplifiers and guitars lacked the top-end produced by high-quality equipment. To overcome this, small battery-powered treble boosters were used. 

Tremolo - a rhythmic pulsing effect obtained by modulating the volume of the signal. It was built into many early combos, being relatively easy to engineer with valve circuitry, and could give anything from a fast rippling sound to a deep throbbing effect. Note: Fender always called their tremolo effect ?vibrato? - this is technically incorrect (see vibrato). 

V
Vibrato - this effect is achieved by modulating the pitch of a signal. The sounds produced can vary from a subtle enhancement to an extreme variation. Early valve combos (such as the Vox AC30) offered vibrato as well as tremolo; however, the feature is more often seen on chorus pedals or digital multi-effects units. 

Volume pedal - a passive device that allows the player to vary the volume of their instrument while performing. Its main use (apart from altering overall volume level) is as a ?swell? pedal; a particularly attractive effect may be achieved by eliminating the percussive attack at the beginning of notes and chords letting them ?float in?. Also works well with string-bends and harmonics. 

W
Wah-wah pedal - a foot operated tone control which became popular in the late 1960?s. When the pedal is flat, a high-treble sound is produced; raising the pedal gradually increases the bass sound. It can be used in several different ways: rocking it gently backwards and fords while playing produces a ?talking guitar? effect or a soft wah sound, while a fast, chopping effect is used by many funk players. It can also be set to an in-between, position, to select a certain tone. MIDI-controlled, rack-mounted auto wah devices are also available. 


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