piano scale
B♭ Mixolydian
B♭1C2D3E♭4F5G6A♭♭7
Major with a flattened seventh: the scale that fits a dominant seventh chord without altering it.
The same sounds spelled from the other side are A♯ Mixolydian.
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
B♭WCWDHE♭WFWGHA♭WB♭
What is true of it
B♭ Mixolydian is B♭, C, D, E♭, F, G and A♭, and back to B♭.
Its steps are W – W – H – W – W – H – W, where W is a whole step and H a half.
On the same tonic, B♭ major differs from it by exactly one note.
Chords made only of these notes
CmminorCm7minor seventhDdimdiminishedDm7♭5half-diminished seventhE♭majorE♭maj7major seventhFmminorFm7minor seventh
Found by testing every chord in the catalogue against these 7 sounds, not chosen.