piano scale
B♭ major
B♭1C2D3E♭4F5G6A7
The seven-note scale everything else is measured against: two whole steps, a half, three whole steps, a half.
The same sounds spelled from the other side are A♯ major.
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♭WFWGWAHB♭
What is true of it
B♭ major is B♭, C, D, E♭, F, G and A, and back to B♭.
Its steps are W – W – H – W – W – W – H, where W is a whole step and H a half.
On the same tonic, B♭ melodic minor differs from it by exactly one note.
Chords made only of these notes
Found by testing every chord in the catalogue against these 7 sounds, not chosen.