piano scale
A♯ Phrygian
A♯1B♭2C♯♭3D♯4E♯5F♯♭6G♯♭7
Minor with a flattened second — the half step right above the tonic is the whole character.
The same sounds spelled from the other side are B♭ Phrygian.
Written out
Every accidental is written in, rather than left to a key signature, so the letters can be read one at a time.
The step pattern
A♯HBWC♯WD♯WE♯HF♯WG♯WA♯
What is true of it
A♯ Phrygian is A♯, B, C♯, D♯, E♯, F♯ and G♯, and back to A♯.
Its steps are H – W – W – W – H – W – W, where W is a whole step and H a half.
It costs 6 alterations to write — near the limit of what a key signature can carry.
On the same tonic, A♯ natural minor differs from it by exactly one note.
Chords made only of these notes
C♯majorC♯7dominant seventhD♯mminorD♯m7minor seventhF♯majorF♯maj7major seventhG♯mminorG♯m7minor seventh
Found by testing every chord in the catalogue against these 7 sounds, not chosen.