Upper structure triad
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An upper structure is a type of chord voicing for dominant seventh chords consisting of a triad over a tritone. The tritone gives the chord the dominant seventh quality, while the triad adds "color tones." For this reason, upper structures give a rich and dense sound.
For example, suppose you wanted to voice a C7 chord on the piano with an upper structure. In the octave just below middle C, play E-Bb with your left hand. You now have several options for your right hand. If you play a root position Eb triad with your right hand, you have added the #9 (Eb), along with the 5 (G) and the 7 (Bb). Similarly, if you play an F#m triad in second inversion, you have added the b9 (C#), the #11 (F#), and the 13 (A).
This example illustrates one of the key advantages of using upper structures: it is much easier to think of "F#m over C7" than "C7 b9 #11 13".
Upper structures are useful in comping for a soloist, but there are several things to watch out for. First, upper structures may be way too complicated for a sensitive moment in a ballad or a pedal point. Furthermore, it is important to pay attention to which dominant alterations the soloist is using. For instance, if the soloist plays D on a C7 chord, the natural 9, the "F#m over C7" upper structure would probably not be appropriate, since it contains the b9.
Most jazz pianists use a wide range of chord voicings, including ones that are less harmonically complicated than upper structures, such as shell voicings or rootless left-hand voicings with octaves in the right hand.