piano scale
D♭ Mixolydian
D♭1E♭2F3G♭4A♭5B♭6C♭♭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 C♯ 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
D♭WE♭WFHG♭WA♭WB♭HC♭WD♭
What is true of it
D♭ Mixolydian is D♭, E♭, F, G♭, A♭, B♭ and C♭, and back to D♭.
Its steps are W – W – H – W – W – H – 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, D♭ major differs from it by exactly one note.
Chords made only of these notes
D♭majorD♭7dominant seventhE♭mminorE♭m7minor seventhFdimdiminishedFm7♭5half-diminished seventhG♭majorG♭maj7major seventh
Found by testing every chord in the catalogue against these 7 sounds, not chosen.