En música y teoría musical, un acorde consiste en un conjunto de tres o más notas diferentes que suenan simultáneamente y que constituyen una unidad armónica. En determinados contextos, un acorde también puede ser percibido como tal aunque no suenen todas sus notas. Pueden formarse acordes con las notas de un mismo instrumento o con notas de diferentes instrumentos (incluyendo la voz humana) tocados a la vez. Formalmente, un acorde posee entre tres y siete notas de las doce que componen una octava; las notas pueden pertenecer a la misma o a diferentes octavas. La distancia entre dos notas musicales se conoce como intervalo musical; los intervalos musicales, combinados, determinan los diferentes tipos de acordes. Cada tipo de acorde puede presentar como tono fundamental cualquiera de las doce notas musicales (do, do♯, re, mi♭, mi, fa, fa♯, sol, la♭, la, si♭, si). Este tono fundamental (también conocido como «nota fundamental», «fundamental», «nota tónica» o «tónica») determina la tonalidad del acorde y constituye la referencia para sus intervalos. Las combinaciones de dos notas suelen llamarse díadas, las de tres, tríadas, las de cuatro, tetradas, las de cinco, quintíadas, las de seis, sextíadas, y las de siete, septíadas. A una sucesión de acordes se la denomina progresión armónica. En una canción, o un tema instrumental, las progresiones armónicas determinan en líneas generales el camino que debe seguir la música de acompañamiento y, el que, en bastante medida, suele seguir la melodía principal, que por razones de armonía debe y suele adaptarse en cierto grado al fondo musical. Además, constituyen normalmente un elemento invariable entre diferentes versiones o variaciones, con los mismos o con diferentes instrumentos, de un mismo tema musical. Por ello, es típico «resumir» el acompañamiento de una canción en sus acordes básicos para luego poder interpretarla libremente al piano o la guitarra con dichos acordes como guía, sin perder la esencia de la composición, es decir, sin que esta deje de ser identificable. En sentido inverso, también es típico componer una canción para uno o varios instrumentos partiendo de la creación de una progresión de acordes. Los acordes más complejos, de 5 o más notas, se utilizan con frecuencia, además de en la música orquestal, en géneros musicales como la canción melódica contemporánea, y, con especial frecuencia, en el jazz.

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A chord, in music, is any harmonic set of usually three or more notes (also called "pitches") that is heard as if sounding simultaneously. In everyday use by musical ensembles such as bands and orchestras, the three or more notes of a chord are often sounded together. However, the notes of a chord do not have to be played together at the same time: arpeggios and broken chords (these involve the notes of the chord played one after the other, rather than at the same time) may, for many practical and theoretical purposes, also constitute chords. Chords and sequences of chords are frequently used in modern West African, Oceanian and Western classical music and Western popular music, whereas they are absent from the music of many other parts of the world. In tonal Western classical music (music with a tonic key or "home key"), the most frequently encountered chords are triads, so called because they consist of three distinct notes: the root note, a third above the root note and a fifth interval above the root note. Further notes may be added to give tetrads such as seventh chords, of which the most commonly encountered example being the dominant seventh chord, which consists of a root note, the major third, the perfect fifth, and the minor seventh above the root. Other chords with more than three notes include added tone chords, extended chords and tone clusters, which are used in contemporary classical music, jazz and other genres. Triads commonly found in the Western classical tradition are major and minor chords, with augmented and diminished chords appearing less often. The descriptions major, minor, augmented, and diminished are referred to collectively as chordal quality. Chords are also commonly classified by their root note—for instance, a C major triad consists of the pitch classes C, E, and G. A chord retains its identity if the notes are stacked in a different way vertically; however, if a chord has a note other than the root note as the lowest note, the chord is said to be in an inversion (this is also called an "inverted chord"). While most chords have at least three notes, power chords, which are widely used in rock music, particularly in hard rock and heavy metal music, have just two notes: the root and the fifth (although the root may be doubled with the octave above). An ordered series of chords is called a chord progression. One example of a widely used chord progression in Western traditional music and blues is the 12 bar blues progression, the simplest versions of which include tonic, subdominant and dominant chords (this system of naming chords is described later in this section). Although any chord may in principle be followed by any other chord, certain patterns of chords are more common in Western music, and some pattern have been accepted as establishing the key (tonic note) in common-practice harmony–notably the movement between tonic and dominant chords. To describe this, Western music theory has developed the practice of numbering chords using Roman numerals which represent the number of diatonic steps up from the tonic note of the scale. Common ways of notating or representing chords in Western music other than conventional staff notation include Roman numerals, figured bass, macro symbols (sometimes used in modern musicology), and chord charts. Each of these systems is more likely to appear in certain contexts: figured bass notation was used prominently in notation of Baroque music, macro symbols are used in modern musicology, and chord charts are typically found in the lead sheets used in popular music and jazz. The chords in a song or piece are also given names which refer to their function. The chord built on the first note of a major scale is called the tonic chord (colloquially called a "I" or "one" chord). The chord built on the fourth note of a major scale is called the subdominant chord (colloquially called a "IV" chord or "four" chord). The chord built on the fifth degree of the major scale is called the dominant chord (colloquially called a "V chord" or "five" chord). There are names for the chords built on every note of the major scale. Chords can be played on many instruments, including piano, pipe organ, guitar and mandolin. Chords can also be performed when multiple musicians play together in a musical ensemble or when multiple singers sing in a choir and they play or sing three or more notes at the same time.

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