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Number of notes

Two-note combinations, whether referred to as chords or intervals, are called dyads. In the context of a specific section in a piece of music, dyads can be heard as chords if they contain the most important notes of a certain chord. For example, in a piece in C Major, after a section of tonic C Major chords, a dyad containing the notes B and D sounds to most listeners as a first inversion G Major chord. Other dyads are more ambiguous, an aspect that composers can use creatively. For example, a dyad with a perfect fifth has no third, so it does not sound major or minor; a composer who ends a section on a perfect fifth could subsequently add the missing third. Another example is a dyad outlining the tritone, such as the notes C and F# in C Major. This dyad could be heard as implying a D7 chord (resolving to G Major) or as implying a C diminished chord (resolving to Db Major). In unaccompanied duos for two instruments, such as flute duos, the only combinations of notes that are possible are dyads, which means that all of the chord progressions must be implied through dyads, as well as with arpeggios.

Chords constructed of three notes of some underlying scale are described as triads. Chords of four notes are known as tetrads, those containing five are called pentads and those using six are hexads. Sometimes the terms trichord, tetrachord, pentachord, and hexachord are used—though these more usually refer to the pitch classes of any scale, not generally played simultaneously. Chords that may contain more than three notes include pedal point chords, dominant seventh chords, extended chords, added tone chords, clusters, and polychords.

Polychords are formed by two or more chords superimposed. Often these may be analysed as extended chords; examples include tertian, altered chord, secundal chord, quartal and quintal harmony and Tristan chord. Another example is when G (G–B–D–F–A ♭ –C ♯) is formed from G major (G–B–D) and D ♭ major (D ♭ –F–A ♭). A nonchord tone is a dissonant or unstable tone that lies outside the chord currently heard, though often resolving to a chord tone.