key signature
A major
A1B2C♯3D4E5F♯6G♯7
3 sharps, written F♯ C♯ G♯
The chords that live in it
Adegree 1 · majorBmdegree 2 · minorC♯mdegree 3 · minorDdegree 4 · majorEdegree 5 · majorF♯mdegree 6 · minorG♯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
A major has 3 sharps.
They are written in this order, always: F♯ C♯ G♯.
Its notes are A, B, C♯, D, E, F♯ and G♯.
Relative: F♯ minor has the same 3 sharps. Parallel: A minor has no sharps and no flats, which is why changing mode changes the signature by three.
Every key
C♭ majorG♭ majorD♭ majorA♭ majorE♭ majorB♭ majorF majorC majorG majorD majorE majorB majorF♯ majorC♯ majorA♭ minorE♭ minorB♭ minorF minorC minorG minorD minorA minorE minorB minorF♯ minorC♯ minorG♯ minorD♯ minorA♯ minor