key signature
C♯ major
C♯1D♯2E♯3F♯4G♯5A♯6B♯7
7 sharps, written F♯ C♯ G♯ D♯ A♯ E♯ B♯
The chords that live in it
C♯degree 1 · majorD♯mdegree 2 · minorE♯mdegree 3 · minorF♯degree 4 · majorG♯degree 5 · majorA♯mdegree 6 · minorB♯dimdegree 7 · diminished
Built by stacking thirds inside the scale and asking what quality comes out, rather than from a table of roman numerals.
What is true of it
C♯ major has 7 sharps.
They are written in this order, always: F♯ C♯ G♯ D♯ A♯ E♯ B♯.
Its notes are C♯, D♯, E♯, F♯, G♯, A♯ and B♯.
Relative: A♯ minor has the same 7 sharps. Parallel: C♯ minor has 4 sharps, which is why changing mode changes the signature by three.
Every key
C♭ majorG♭ majorD♭ majorA♭ majorE♭ majorB♭ majorF majorC majorG majorD majorA majorE majorB majorF♯ majorA♭ minorE♭ minorB♭ minorF minorC minorG minorD minorA minorE minorB minorF♯ minorC♯ minorG♯ minorD♯ minorA♯ minor