Root, third and seventh: the guide tones fix the quality, so the fifth is dropped. That is what leaves a string free, so pick a tension to put on top.
BbmMaj7b5 is Bb, Db, Fb and A — Bbm7b5 with the seventh raised a semitone.
It sounds brittle and impossible to mistake. Scarce enough that most players meet it as a passing sonority rather than as a chord they chose.

| Bb | Db | Fb | A |
|---|---|---|---|
| 1 | ♭3 | ♭5 | 7 |
Rare tonic — a half-diminished chord with its seventh raised instead.
Also written BbmM7b5 or Bbm(maj7)b5. The same chord either way; which spelling you meet depends on who wrote the chart.
Root, third and seventh. The third and seventh, the guide tones, are what make the chord major, minor or dominant, so the fifth can go: it says nothing the symbol needs.
Three notes means a spare string, which is the real point. Put a tension on it.
BbmMaj7b5 has 4 of them, on strings 5-3-2-1, strings 6-4-3-2 and the bottom four strings, from open position up to the 12th — all in one inversion, so the bass note never moves. The one to learn first is the Bb shape at the nut.
Start with x-x-8-9-10-9 — no barre. 1 of the 6 shapes here sits in open position. 3 of the 6 need a barre. Together they cover the neck from the 1st fret to the 14th.
| Frets | Where | Bass | Top | Hand |
|---|---|---|---|---|
| x12220 | open position | Bb, root position | Fb | barred |
| 67x655 | 5th fret | Bb, root position | A | all fretted |
| 6776x6 | 6th fret | Bb, root position | Bb | barred |
| x-x-8-9-10-9 | 8th fret | Bb, root position | Db | all fretted |
| x-13-x-14-14-12 | 12th fret | Bb, root position | Fb | all fretted |
| x-13-14-14-14-x | 13th fret | Bb, root position | Db | barred |
The same notes rebuilt from scratch give 13 drop 2, 9 drop 3 and 1 open string voicings, each on its own page.
Tensions. A shell of BbmMaj7b5, root, third and seventh, leaves a string free, and the tones that belong over it are the 11 (Eb) and the b13 (Gb).
Every note of BbmMaj7b5 is in Bb diminished.
Change one note of BbmMaj7b5 and you have Bbdim (drop the major seventh), Bbdim7 (the major seventh becomes a sixth), Bbmaj7b5 (the minor third becomes a major third), Bbm7b5 (the major seventh becomes a flat seventh) or BbmMaj7 (the flat fifth becomes a fifth).
BbmMaj7b5 is a diminished chord — root, minor third, flat fifth. Diminished and half-diminished chords on every root.
On BbmMaj7b5’s sound and use this page follows Robert Rawlins & Nor Eddine Bahha, Jazzology (Hal Leonard, 2005). The shapes and the derived voicings come from different places, and both are explained in where the chords and the theory come from.
Shows each chord the way this style would voice it, and marks which of its alternatives belong to the idiom.