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.
Current practice is to insert enough naturals to cancel the previous key signature when moving into the open key. Otherwise just put the new key signature.