Rootless voicings (also called "left-hand voicings") were systematised by Mark Levine in The Jazz Piano Book (pp. 41-50). The Type A and Type B positions, and the practice of alternating between them for smooth voice leading, originate from this text.
Rootless voicings drop the root (the bassist plays it) and add the 9th for colour. Type A stacks 3rd-5th-7th-9th from the bottom. Type B rearranges the same notes with the 7th on the bottom: 7th-9th-3rd-5th. Alternating A and B in a progression gives you the smoothest possible voice leading.
Type A: 3rd + 5th + 7th + 9th (ascending). Type B: 7th + 9th + 3rd + 5th (7th dropped an octave below the 3rd).
Your primary voicing when playing with a bassist. The A/B alternation system is what professional pianists use in combo settings. Essential for ii-V-I progressions — the 7th of one chord resolves stepwise to the 3rd of the next. This is the Bill Evans innovation.
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