piano scale
C♯ major
C♯1D♯2E♯3F♯4G♯5A♯6B♯7
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 D♭ 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
C♯WD♯WE♯HF♯WG♯WA♯WB♯HC♯
What is true of it
C♯ major is C♯, D♯, E♯, F♯, G♯, A♯ and B♯, and back to C♯.
Its steps are W – W – H – W – W – W – H, where W is a whole step and H a half.
It costs 7 alterations to write — near the limit of what a key signature can carry.
On the same tonic, C♯ melodic minor differs from it by exactly one note.
Chords made only of these notes
C♯majorC♯maj7major seventhD♯mminorD♯m7minor seventhF♯majorF♯maj7major seventhG♯majorG♯7dominant seventh
Found by testing every chord in the catalogue against these 7 sounds, not chosen.