A power chord Play^ⓘ, also called a fifth chord, is a colloquial name for a chord on guitar, especially on electric guitar, that consists of the root note and the fifth, as well as possibly octaves of those notes. Power chords are commonly played with an amp with intentionally added distortion or overdrive effects. Power chords are a key element of many styles of rock, especially heavy metal and punk rock.

When two or more notes are played through a distortion process that non-linearly transforms the audio signal, additional partials are generated at the sums and differences of the frequencies of the harmonics of those notes (intermodulation distortion). When a typical chord containing such intervals (for example, a major or minor chord) is played through distortion, the number of different frequencies generated, and the complex ratios between them, can make the resulting sound messy and indistinct. This effect is accentuated as most guitars are tuned based on equal temperament, with the result that minor thirds are narrower, and major thirds wider, than they would be in just intonation.

However, in a power chord, the ratio between the frequencies of the root and fifth are very close to the just interval 3:2. When played through distortion, the intermodulation leads to the production of partials closely related in frequency to the harmonics of the original two notes, producing a more coherent sound. The intermodulation makes the spectrum of the sound expand in both directions, and with enough distortion, a new fundamental frequency component appears an octave lower than the root note of the chord played without distortion, giving a richer, more bassy and more subjectively "powerful" sound than the undistorted signal. Even when played without distortion, the simple ratios between the harmonics in the notes of a power chord can give a stark and powerful sound, owing to the resultant tone (combination tone) effect. Power chords also have the advantage of being relatively easy to play , allowing fast chord changes and easy incorporation into melodies and riffs.

Theorists are divided on whether a power chord can be considered a chord in the traditional sense, with some requiring a "chord" to contain a minimum of three degrees of the scale. When the same interval is found in traditional and classical music, it would not usually be called a "chord", and may be considered a dyad (separated by an interval). However, the term is accepted as a pop and rock music term, most strongly associated with the overdriven electric guitar styles of hard rock, heavy metal, punk rock, and similar genres. The use of the term "power chord" has, to some extent, spilled over into the vocabulary of other instrumentalists, such as keyboard and synthesizer players.

Power chords are most commonly notated 5 or (no 3). For example, "C5" or "C(no 3)" refer to playing the root (C) and fifth (G). These can be inverted, so that the G is played below the C (making an interval of a fourth). They can also be played with octave doublings of the root or fifth note, which makes a sound that is subjectively higher pitched with less power in the low frequencies, but still retains the character of a power chord.

Another notation is ind, designating the chord as "indeterminate". This refers to the fact that a power chord is neither major nor minor, as there is no third present. This gives the power chord a chameleon-like property; if played where a major chord might be expected, it can sound like a major chord, but when played where a minor chord might be expected, it can sound minor.

The first written instance of a power chord for guitar in the 20th century is to be found in the "Preludes" of Heitor Villa-Lobos, a Brazilian composer of the early twentieth century. Although classical guitar composer Francisco Tárrega used it before him, modern musicians use Villa-Lobos's version to this day. Power chords' use in rock music can be traced back to commercial recordings in the 1950s. Robert Palmer pointed to electric blues guitarists Willie Johnson and Pat Hare, both of whom played for Sun Records in the early 1950s, as the true originators of the power chord, citing as evidence Johnson's playing on Howlin' Wolf's "How Many More Years" (recorded 1951) and Hare's playing on James Cotton's "Cotton Crop Blues" (recorded 1954). Scotty Moore opened Elvis Presley's 1957 hit "Jailhouse Rock" with power chords. The "power chord" as known to modern electric guitarists was popularized first by Link Wray, who built on the distorted electric guitar sound of early records and by tearing the speaker cone in his 1958 instrumental "Rumble."

A later hit song built around power chords was "You Really Got Me" by the Kinks, released in 1964. This song's riffs exhibit fast power-chord changes. The Who's guitarist, Pete Townshend, performed power chords with a theatrical windmill-strum, for example in "My Generation". On King Crimson's Red album, Robert Fripp thrashed with power chords. Power chords are important in many forms of punk rock music, popularized in the genre by Ramones guitarist Johnny Ramone. Many punk guitarists used only power chords in their songs, most notably Billie Joe Armstrong and Doyle Wolfgang von Frankenstein.