B7b5 is B, D#, F and A — with the fifth lowered a semitone.
It sounds symmetrical and slightly unreal. The same four notes spell a 7♭5 a tritone away, which is exactly why one can stand in for the other.
Altered dominant — and the chord that makes the tritone substitution work.
Also written B7(b5), B7-5 or B7♭5.
Built on the flat sixth of a minor key, B7b5 is the classical French sixth under a lead-sheet name, one of the strongest ways into A#: its bass falls a half step and its seventh rises a half step, both onto the V’s root. The 7♭5 a tritone away has the same four notes.
Listen for: The seventh rises a half step as the bass falls a half step; they meet on the root of the V, an octave apart.
Heard in“Bernie’s Tune”
Every note of B7b5 is in B whole tone, B lydian dominant, B altered and B half-whole diminished.
Each of those is a page: the scale across the neck, and the chords it harmonises into — which is where B7b5 sits in it. Every scale.
Change one note of B7b5 and you have (the flat seventh becomes the major seventh), (the major third becomes the minor third), (the flat fifth becomes the fifth), (the flat fifth becomes the sharp fifth) or (add the ninth).
B7b5 is an altered chord — a dominant seventh with its fifth or ninth moved. Altered dominant 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.