The chords that live in it
Cdegree 1 · majorDmdegree 2 · minorEmdegree 3 · minorFdegree 4 · majorGdegree 5 · majorAmdegree 6 · minorBdimdegree 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 no sharps and no flats.
It is the one key whose signature is empty, which is why every explanation of every other key starts here.
Its notes are C, D, E, F, G, A and B.
Relative: A minor has the same no sharps and no flats. Parallel: C minor has 3 flats, which is why changing mode changes the signature by three.
Every key
C♭ majorG♭ majorD♭ majorA♭ majorE♭ majorB♭ majorF majorG majorD majorA majorE majorB majorF♯ majorC♯ majorA♭ minorE♭ minorB♭ minorF minorC minorG minorD minorA minorE minorB minorF♯ minorC♯ minorG♯ minorD♯ minorA♯ minor