Adim7 is A, C, Eb and Gb — with a diminished seventh stacked on top.
It sounds suspenseful and rootless. Three stacked minor thirds divide the octave evenly, so the same four notes spell four dim7 chords and the shape repeats every three frets.
Leading-tone or passing chord — it can resolve almost anywhere.
Also written A°7, Ao7 or A diminished 7.
Adim7 shares its root with A and slips in between D and the I. It isn’t a new harmony, only decoration: two notes hold, and the rest move by step into the home chord.
Listen for: The IV’s root rises a half step, then another, into the fifth of the I, over a bass that never moves.
Heard in“True Love”“Calendar Girl”“Daydream”
Adim7 is the dominant seventh flat nine of the chord a half step above it, with the root left out. Any of its four notes can be the one that rises, so it can lead into four chords; between Abmaj7 and Bbm7 it walks the bass up by half steps.
Listen for: The bass climbs a half step at a time, from the root of the I to the root of the ii.
Adim7 occurs naturally in B♭ minor: the harmonic minor scale builds it on the leading tone, a half step below Bbm.
Played after Amaj7, Adim7 can lead into Bbmaj7, and so into a new key a half step up. Since any of its notes can act as the leading tone, one diminished chord can lead into four keys.
Alternating Bb6 and Adim7 is the sixth–diminished sound: the diminished chord is the V7♭9 of Bb6 with its root left out.
Every note of Adim7 is in A diminished and A half-whole diminished.
Each of those is a page: the scale across the neck, and the chords it harmonises into — which is where Adim7 sits in it. Every scale.
Change one note of Adim7 and you have (drop the diminished seventh), (the flat fifth becomes the fifth), (the diminished seventh becomes the flat seventh), (the diminished seventh becomes the major seventh) or (the flat fifth becomes the fifth).
Adim7 is a diminished chord — root, minor third, flat fifth. Diminished and half-diminished 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.