Left Hand Shells 🐚
Two notes in the left hand, and the right hand free.
What a shell is
A shell is the root and one other note. That is the whole left hand. Bud Powell comped and soloed over one for a career, and so did most of the players who came after him.
Two notes leave room. The bass player has the bottom, the right hand has the melody, and nothing in the middle fights either one.
The three shapes
Root and seventh, written 1-7. Root and third, written 1-3. Root and third an octave higher, written 1-10.
The seventh and the third are the ones in the chord you are playing. A dominant or a minor chord gives a flat seventh, a major 7 chord gives the natural seventh, and a diminished chord gives its own diminished seventh. The third is major on a dominant chord and minor on a minor chord.
A 6 chord has no seventh, so the wide shape takes the sixth instead and reads 1-6. C6 is C and A. That is the shape under most of the bebop tonic chords.
Fingering
The wide shapes, 1-7 and 1-10, take the little finger on the root and the thumb on top. The narrow one, 1-3, takes the index finger on the root and the thumb on top. Learning those two hand positions is most of the work.
Where it sits
The upper note is the tenor, and it lives in the octave below middle C. The root falls where the shape puts it, near the bottom of the bass clef for a tenth and higher up for a seventh. Keep the tenor in that octave and the shell never turns to mud.
The tenor is the line
The tenor is the note to voice lead. Move it as little as the next chord allows and the shapes choose themselves. In C the 2-5-1 runs D and C, then G and B, then C and A. The seventh of the 2 chord falls a half step to the third of the 5, which is the oldest move in the music, and the line keeps falling to the sixth of the 1.
That is why the shapes matter. When the seventh is in reach you play 1-7, when the third is closer you play 1-3, and the line stays smooth either way.
The progressions
The widget below runs the stock progressions. The engine picks the shell that keeps the tenor closest, and the menu on each card overrides it, so you can hear the line go the other way. The Major chords switch turns every 6 chord into a major 7, which moves the tenor of the tonic from the sixth up to the seventh.
The Song menu swaps the stock progression for a whole tune. Pick one and watch the tenor: it walks down through a chorus, and where the engine has to jump it back up is where you would reset.
How to practice
Play each progression in all twelve keys, around the circle of fourths. Say the tenor note out loud as you play it. Once the line is under your hand, put the four-note right hand from Trombone Voicings on top and hear the two posts meet.
Then take it to a tune. Open a fake book, find something with plenty of 2-5-1s in it, and play the shells straight off the chord symbols.
The tenor runs out of room. Every 2-5 pulls it down a little, and after a few of them it is sitting at the bottom of its octave with nowhere left to fall. Jump it back up an octave and carry on. Every player does this, and it is worth practicing on purpose.
Where you reset matters more than how often. Do not break the line inside a cadence. Wait until a new 2-5-1 starts and reset on its first chord, where the ear is expecting a fresh start anyway.