In jazz, 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 minor 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 (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 9th, augmented 11th, and major 13th):
In relation to the root of C, the C♯ (enharmonic with D♭) functions as the minor 9th, the F♯ as the augmented 11th, and the A as the major 13th, respectively.
Determining which additional pitches can be juxtaposed with the chord is achieved by considering the relationship between a particular chord and the scale it implies. An example follows:
Common jazz parlance refers to upper structures by way of the interval between the root of the bottom chord and the root of the triad juxtaposed above it. For instance, in example one above (C7♯9) the triad of E♭ major is a (compound) minor 3rd away from C (root of the bottom chord). Thus, this upper structure can be called upper structure flat three, or US♭III for short.
Other possible upper structures are:
The second item in the list above (C7♭9♯11) has a related version called upper structure sharp four minor--with the written shorthand US♯iv--created with an F♯ minor triad. (See "Example 2" above.)