Bbm7b5 has two useful names. “Minor seven flat five” describes Bb, Db, Fb and Ab; “half-diminished” distinguishes its flat seventh from a diminished seventh.
Its is the unsettled voice inside an otherwise familiar minor seventh frame. In a minor ii–V, that frame normally hands the phrase to V.
Also written Bbø, Bbø7, Bbm7-5, Bbm7(b5) or Bb half diminished.
In a minor key the ii chord is half-diminished, and Bbm7b5 leads to the V and on to the i. Its flat fifth is the key’s flat sixth: it holds as the dominant’s flat nine, then falls to the fifth of the home chord, whether that chord is minor or major.
Listen for: The flat fifth holds over the barline as the dominant’s flat nine, then falls a half step to the fifth.
Heard in“There Will Never Be Another You”“Night and Day”, resolving to major
In a major key Bbm7b5 is built on the leading tone and does the dominant’s job. Lower its seventh a whole step and it becomes F#7; its bass, the leading tone, then rises into the root of B.
Listen for: Only the seventh moves at first, down a whole step, and the chord is the V7. That note then holds as the fifth of the I.
In the basic turnaround in C♯ minor, Bbm7b5 is the second chord, between the i and the ii.
A turnaround in G♭ major can start on the iii instead of the I, and that iii can be played as Bbm7b5, in front of an altered VI7.
In which keys. Every note of Bbm7b5 belongs to Ab minor and B major.
Contains Bbm7b5 and adds eleventh and fifth.
Contains Bbm7b5 and adds flat ninth, eleventh and sharp fifth.
Contains Bbm7b5 and adds flat ninth, major third and sharp fifth.
Contains Bbm7b5 and adds flat ninth, major third, fifth and thirteenth.
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.