Key of C Key Signature has no symbol, so say I was notating music in G Major then wanted to change to the Key of C, how would I show this?
Removing accidentals in the key signature is done with the natural symbol. Put a natural symbol on the position of each accidental you want to cancel, and reiterate the ones that are left.
In the above:
- Cmaj cancels all 3 sharps of Amaj.
- Gmaj cancels 2 sharps of Amaj and reiterates the one left.
- Fmaj cancels the sharp of Gmaj and notes the new flat.
As Tim notes in his answer,
It's normal to cancel the previous key signature, so a natural sign (♮) on the F line(s) will be sufficient.
Going the other way (i.e. from C to G) will require merely a ♯ sign on the 'F' line. With more ♯s or ♭s, the new key signature will have naturals cancelling the previous unwanted ♯ or ♭ — a weird key signature with more info than usual!
I tend to put a double barline (both thin) at that point, as it's often the start of a new part anyway, but also to point out the key change clearly … especially in C to something else, or something else to C. Obviously going from lots of ♯s/♭s to C will show up as lots of naturals, though.