Chords

In music and music theory a chord is a set of three or more different notes from a specific key that sound simultaneously. Chords constructed of three notes are described as triads and consist of two intervals. The technical name for triad chords is tertian sonorities and is understood to be chords constructed from stacks of thirds relative to some underlying scale. Two-note combinations are commonly referred to as intervals. The technical name for an interval is dyads. A succession of chords is called a chord progression.

The word chord comes from cord which is a Middle English shortening of accord. In the Middle Ages, Western harmony featured the perfect intervals of a fourth, a fifth, and an octave. In the 15th and 16th centuries, the major and minor triads (see below) became increasingly common, and were soon established as the default sonority for Western music. Four-note "seventh chords" were then widely adopted from the 17th century. The harmony of many contemporary popular Western genres continues to be founded in the use of triads and seventh chords, though far from universally. Notable exceptions include: modern jazz (especially circa 1960), in which chords often include at least five notes, with seven (and occasionally more) being quite common; and atonal or post-tonal contemporary classical music (including the music of some film scores), whose chords can be far more complex, rooted in such disparate harmonic philosophies that traditional terms such as triad are rarely useful.
Chords are so well-established in Western music that sonorities of two pitches, or even monophonic melodies, are often interpreted by listeners (musicians and non-musicians alike) as "implying" chords. This psychoacoustic phenomenon occurs as a result of a lifetime of exposure to the conventional harmonies of music, with the result that the brain "supplies" the complete expected chord in its absence. [2]
Composers can and do take advantage of this tendency to surprise the listener, by deliberately avoiding certain defining tones. For instance, a composition may be predominantly composed in the pentatonic minor scale, implying common Aeolian mode to the listener, before deliberately including a more uncommon tone in a melodic progression or chord, such as a major VI (signalling Dorian mode) or a flattened II (signalling Phrygian mode).