Guide Tones I

The Guide Tones I voicing of Bbmaj9 stacks D and A from the bottom up: the major 3rd and major 7th of the chord. It leaves out the root, Bb, for the left hand or the bass player to take.

It's one of the Guide Tones voicings: just the 3rd and 7th, the notes that define each chord, played over the root. The other for Bbmaj9 is Guide Tones II.