Chord and nonchord tones are defined by their membership in a chord: "The pitches which make up a chord are called chord-tones: any other pitches are called non-chord-tones. They are also defined by the time at which they sound: "Nonharmonic tones are pitches that sound along with a chord but are not chord pitches.".
For example, if a piece of music is currently on a C Major chord, the notes CEG are members of that chord, while any other note played at that time is a nonchord tone. Such tones are most obvious in homophonic music but also often occur in contrapuntal music.
"Most nonharmonic tones are dissonance and create intervals of a second, fourth or seventh," which are required to resolve to a chord tone in conventional ways. If the note fails to resolve until the next change of harmony, it may instead create a seventh chord or extended chord. While it is theoretically possible that for a three-note chord there are (in equal temperament) nine possible nonchord tones, nonchord tones are usually in the prevailing key. Augmented and diminished intervals are also considered dissonant, and all nonharmonic tones are measured from the bass, or lowest note sounding in the chord except in the case of nonharmonic bass tones.
Nonharmonic tones generally occur in a pattern of three pitches, of which the nonharmonic tone is the center:
1 - 2 - 3
Preceding tone - Nonharmonic tone - Following tone
(chord tone) - - (chord tone)
Preparation - Dissonance - Resolution
Nonchord tones are distinguished through how they are used. The most important distinction is whether they occur on a strong or weak beat and are thus accented or unaccented. They are also distinguished by their direction of approach and departure and the voice or voices in which they occur, and the number of notes they contain.
Over time some nonchord tones supposedly became chord tones, such as the seventh in a seventh chord. In European classical music "The greater use of dissonance from period to period as a result of the dialectic of linear/vertical forces led to gradual normalization of ninth, eleventh, and thirteenth chords[in analysis and theory]; each additional non-chord tone above the foundational triad became frozen into the chordal mass.
In practice and analysis, neighboring tones are often differentiated depending upon whether or not they are lower or higher than the chord tones surrounding them. A neighboring tone that is a step higher than the surrounding chord tones is called an upper neighboring tone or an upper auxiliary note while a neighboring tone that is a step lower than the surrounding chord tones is a lower neighboring tone or lower auxiliary note.
Suspensions may be further described using the number of the interval forming the suspension and its resolution; e.g. 4-3 suspension, 7-6 suspension. Most suspensions resolve downwards; the example shown above, a 7-8 suspension, is a rare example of an upwards resolution (also called a retardation). A suspension must be prepared with the same note (in the same voice) using a chord tone in the preceding chord; otherwise it is an appoggiatura.
Composing a chain of suspensions is the fourth species of counterpoint.
Examples include the Elektra chord.