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.
AmMaj7b5 is A, C, Eb and G# — Am7b5 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.

| A | C | Eb | G# |
|---|---|---|---|
| 1 | ♭3 | ♭5 | 7 |
Rare tonic — a half-diminished chord with its seventh raised instead.
Also written AmM7b5 or Am(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.
AmMaj7b5 has 4 of them, on the middle four strings, strings 6-4-3-2, strings 5-3-2-1 and the bottom four strings, from the 3rd fret up to the 11th — all in one inversion, so the bass note never moves. The one to learn first is the A shape at the 4th fret, which is the easiest grip here.
Start with xx7898 — 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 13th.
| Frets | Where | Bass | Top | Hand |
|---|---|---|---|---|
| x01114 | open position | A, root position | G# | barred |
| 56x544 | 4th fret | A, root position | G# | all fretted |
| 5665x5 | 5th fret | A, root position | A | barred |
| xx7898 | 7th fret | A, root position | C | all fretted |
| x-12-x-13-13-11 | 11th fret | A, root position | Eb | all fretted |
| x-12-13-13-13-x | 12th fret | A, root position | C | barred |
The same notes rebuilt from scratch give 13 drop 2 and 8 drop 3 voicings, each on its own page.
Tensions. A shell of AmMaj7b5, root, third and seventh, leaves a string free, and the tones that belong over it are the 11 (D) and the b13 (F).
Every note of AmMaj7b5 is in A diminished.
Change one note of AmMaj7b5 and you have Adim (drop the major seventh), Adim7 (the major seventh becomes a sixth), Amaj7b5 (the minor third becomes a major third), Am7b5 (the major seventh becomes a flat seventh), AmMaj7 (the flat fifth becomes a fifth) or Am/G# (the flat fifth becomes a fifth).
AmMaj7b5 is a diminished chord — root, minor third, flat fifth. Diminished and half-diminished chords on every root.
On AmMaj7b5’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.