Bbaug is Bb, D and F# — with the fifth raised a semitone.
It sounds stretched, and suspended between keys. Its commonest job is passing between a major triad and its sixth or seventh, with the raised fifth as a single voice walking up.
Passing chord — it belongs to no key in particular.
Also written Bb+, Bb(#5) or Bb augmented.
Bbaug is most often a dominant with its fifth raised. That note becomes a second leading tone, a half step below the third of Eb. It needs a major tonic: a minor third is a whole step away, and the pull is gone.
Listen for: The raised fifth rises a half step to the third of the I. Without it, the same voice has a whole step to climb.
Heard in“School Days” (opening chord)“Oh! Darling”“From Me to You” (end of the bridge)
Over a held Bb, the fifth can climb by half steps: Bbaug, Bb6, Bb7, and on to Eb. Bbaug is just the first step of the line.
Listen for: The fifth climbs a half step at a time, to the sixth and the flat seventh, then settles a half step down onto the third of the IV.
Heard in“Kind of a Drag”“Laughing”“It’s My Party”
Every note of Bbaug is in Bb whole tone, Bb lydian augmented and Bb altered.
Each of those is a page: the scale across the neck, and the chords it harmonises into — which is where Bbaug sits in it. Every scale.
Change one note of Bbaug and you have (the sharp fifth becomes the fifth), (add the major seventh) or (add the flat seventh).
Bbaug is a augmented chord — root, major third, sharp fifth. Augmented 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.