thumb|A C major chord.
A chord is the simultaneous sound of two or more musical notes. The most common chord has three notes and is known as a triad. Added tone chords, extended chords, and tone clusters can have more than three notes and are common in contemporary classical music and jazz.
An arpeggio is a chord where notes are sounded separately. A series of chords is sometimes called a chord progression. There are several ways to notate chords including figured bass, Roman numerals, the Nashville Number System, and alphabetical chord notation.
Definition
thumb|The "Promenade" in [[Modeste Mussorgsky's Pictures at an Exhibition demonstrates a simple homophonic texture. The term derives from "accord", which became "cord" in Middle English. The original meaning was an agreement or harmonious sound. In the 17th century, the spelling was changed to "chord" to prevent confusion with "cord".
Until the Middle Ages, harmony was any combination of two notes. In the Renaissance, the simultaneous sound of three notes began to be understood as the working definition of harmony. An arpeggio is a broken chord where each note is sounded successively instead of simultaneously. Chords can also be implied by melodies.
A chord progression is a collection of harmonies with a specific destination or purpose, such as reinforcing the tonic or modulation to a new pitch center. Chord progressions are common in Western music. Homophonic textures where the melody and harmony generally move in unison are considered the standard practice in classical music and remain central to music instruction. The study of harmony involves chords and progressions and the principles of connection that govern them.
History
thumb|399x399px|A 9th century example of [[polyphony in parallel fifths from Musica enchiriadis: Tu patris sempiternus es filius written in Daseian notation.]]
Between the 16th and 18th centuries, chords developed into primary musical elements that were distinct from their polyphonic origins. In early church polyphony known as organum, a plainsong was paired with another melody. This two-part counterpoint developed gradually into ever more complex polyphonic writing.
In organum's simplest form, the plainchant was doubled in a perfect interval like the fourth, fifth, or octave. Chords were incidental results of the melodic lines.
During the Renaissance, polyphony became more complex. Seventh chords started to appear in the 16th century.
As tonality expanded, structures like Neapolitan and augmented sixth chords used chromatic notes outside of the diatonic scale. Such altered chords complicated voice leading and led to more eclectic cadences and modulations. Composers began to strip the function from such chords and embrace them purely for their sonic qualities. 20th century music greatly expanded chords, adding tones and often dissonance. Jazz has a particularly eclectic chord vocabulary.
Structure
thumb|The C major thirteenth chord can be seen as a [[polychord combination of several different chords.]]Just as scales derive from the harmonic series, the major chord is likewise formed by overtones 4, 5, and 6. The next overtone creates a seventh chord. A common triad has three notes: the root, third, and fifth. The intervals are identified by their distance from the root of the chord.
Continuing up in thirds yields a seventh, ninth, eleventh, and thirteenth before returning to the root again. Complex sonorities larger than a tetrad are sometimes called polychords because they can be seen as combinations of two different chords.
Chords are identified by their intervals. Additional intervals like sevenths are also described by their quality in relation to the root. Chords also shrunk to tone clusters made of small intervals like seconds. The resulting sound mass was a common feature in the 20th century. Microtonal chords are also used by many contemporary composers.