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Shell voicings are simple but functional.
They make use of the root and either the third of the seventh any chord. Sometimes referred to as "shell" voicings, these are commonly used as left-hand accompaniment in the piano stylings of Bud Powell, Horace Silver, Sonny Clark, and others who play primarily in the "be-bop" idiom.
With only two notes, they are harmonically incomplete; however, they do convey enough information to supplement many right hand melodies or improvised lines, especially those in the be-bop style where melodies are crafted to clearly outline the harmony.
Most Jazz pianists play these kind of voicings in a relatively sparse and percussive manner on medium or up-tempo tunes.
Shell voicings are most effective when the top note (played by the thumb) falls between D below middle C and the D next to middle C. These voicings are particularly useful in support an improvised line played in the middle range of the keyboard, which is stylistically typical of be-bop playing. If your melody or improvisation should dip lower and conflict with a voicing, here are some options:
1. Break the voice leading and pick the other inversion (e.g. Root- 3rd instead of Root-7th) which falls in a lower register;
2. For that moment play only the root, a Root-5th, or nothing at all in the left hand; or
3. Play the entire melody up an octave.
When you apply shell voicings to tunes, it is OK to break voice leading occasionally in order to select the inversion which best complements a given melody note (i.e. does not double it.) Since the voicings are likely to be somewhat rhythmically detached from each other, voice leading with shell voicings is not as critical as with other voicings. Nonetheless, it is still always best to avoid breaking voice leading within ii-V and ii-V-I progressions.
Always remember that any Root-3rd structure may be expanded into a Root-10th which sounds fuller.
Whether or not you are able to use a tenth in place of a 3rd depends upon the size of your left hand and spatial distance the 10th covers on the keyboard.
Physically, minor 10ths are easier to reach than major 10ths. In a ii-V progression, it is more natural to close in from a Root-10th voicing to a Root-7th, rather than expanding from a Root-7th to a Root-10th.
You just have to experiment to decide which tenth intervals fit your hands. But always STOP IMMEDIATELY if you experience any hint of pain in stretching a tenth or, for that matter, while playing anything on the piano.
Ron Worthy is a Music Educator, Songwriter and Performer. To learn more about "shell voicings," please visit: http://www.mrronsmusic.com and http://www.playpianotonight.com