piano scale
D♭ major
D♭1E♭2F3G♭4A♭5B♭6C7
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 C♯ 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
D♭WE♭WFHG♭WA♭WB♭WCHD♭
What is true of it
D♭ major is D♭, E♭, F, G♭, A♭, B♭ and C, and back to D♭.
Its steps are W – W – H – W – W – W – H, where W is a whole step and H a half.
On the same tonic, D♭ melodic minor differs from it by exactly one note.
Chords made only of these notes
CdimdiminishedCm7♭5half-diminished seventhD♭majorD♭maj7major seventhE♭mminorE♭m7minor seventhFmminorFm7minor seventh
Found by testing every chord in the catalogue against these 7 sounds, not chosen.