piano scale
B♭ Lydian
B♭1C2D3E♯4F5G6A7
Major with a raised fourth. The fourth mode of the major scale.
The same sounds spelled from the other side are A♯ Lydian.
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♭WCWDWEHFWGWAHB♭
What is true of it
B♭ Lydian is B♭, C, D, E, F, G and A, and back to B♭.
Its steps are W – W – W – H – W – W – H, 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
CmajorC7dominant seventhDmminorDm7minor seventhEdimdiminishedEm7♭5half-diminished seventhFmajorFmaj7major seventh
Found by testing every chord in the catalogue against these 7 sounds, not chosen.