The third voice from the top lowered an octave. The dropped voice becomes the bass, which is why a string is skipped.
Bbmaj9 is Bb, D, F, A and C — Bbmaj7 with the ninth stacked above it.
It sounds a maj7 with more air in it. The ninth argues with nothing here, so it works anywhere a maj7 does.
Tonic — an extended resting chord.

| Bb | D | F | A | C |
|---|---|---|---|---|
| 1 | 3 | 5 | 7 | 9 |
Also written BbM9, BbΔ9, Bbma9 or Bb major 9. The same chord either way; which spelling you meet depends on who wrote the chart.
The same operation one voice further down: the third voice from the top falls an octave and becomes the bass.
That leaves a bigger hole under the top three notes, so a string gets skipped: the muted string in the middle is the shape, not a mistake.
Bbmaj9 has 8 of them, on strings 6-4-3-2 and strings 5-3-2-1, from open position up to the 10th — every inversion the chord has: Bb in the bass, D in the bass, A in the bass and C in the bass. The one to learn first is the A shape at the nut.
4 of them sit on strings 6-4-3-2, so you can run the inversions up one set without changing strings — which is the whole reason to learn them as a family rather than as separate grips.
Five notes and four fingers, so something has to go. Keep D and A (they fix the quality) and C (what the symbol is about). Let F and Bb go, in that order — a perfect fifth says nothing the root has not, and in a band the bass has the root.
Play Bbmaj9 as a shell — root, third and seventh — and the spare string takes the #11 (E) or the 13 (G).
All 12 common shapes as fret numbers, lowest string first. Hover or tap one to see it drawn.
How you get to it. The turn that lands on Bbmaj9 is Cm7 to F7 — the ii and the V of Bb — and F7 shares its third and seventh with Cm7, so the hand barely moves between them.
In which keys. Every note of Bbmaj9 belongs to D minor (the ♭VI), F major (the IV, the subdominant), G minor (the ♭III) and Bb major (the I, the tonic).
Every note of Bbmaj9 is in Bb major and Bb lydian.
Change one note of Bbmaj9 and you have Bbmaj7 (drop the ninth), Bbmaj11 (add the eleventh), Bbmaj13 (add the thirteenth), BbmMaj9 (the major third becomes the minor third), Bbadd9 (drop the major seventh) or Bb69 (the major seventh becomes the sixth).
Bbmaj9 is a major chord — root, major third, fifth. Major chords on every root.
The common shapes and the derived voicings come from different places, and both are explained in how this site is built.
Shows each chord the way this style would voice it, and marks which of its alternatives belong to the idiom.