Upper structure
In jazz music, the term upper structure or upper structure triad refers to a voicing approach developed by jazz pianists and arrangers defined by the sounding of a major or minor triad in the uppermost pitches of a more complex harmony.
Example 1: Below, a common voicing used by jazz pianists is given for the chord C7♯9 (C major chord with a minor 7th, and extended with an augmented 9th).
In the lower-stave the notes E♮ and B♭ are given. These form a tritone which defines the dominant sound, and are the major 3rd and flattened 7th of the C7♯9 chord.
In the upper-stave the notes E♭, G, and B♭ are given together: these form an E♭ major triad.
This E♭ major triad is what would be called the upper structure. Considered in relation to the root C, the notes of this E♭ major triad function, respectively, as the sharpened ninth (actually a flattened tenth, enharmonically equal to the sharpened ninth which forms the root of the E♭ major chord), fifth, and seventh in relation to that root.
(Note: the root C is omitted here, and is often done so by jazz pianists for ease of playing, or because a bass player is present.)
Example 2: The following example illustrates the notes of an F♯ minor triad functioning as part of a C13♭9♯11 chord (C major chord with a minor 7th, minor ninth, augmented 11th, and major 13th):
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