It's called Enharmonic notes: Notes that sound the same but are spelled differently. (Example: C#=Db, F#=Gb, G#=Ab, B#=C, E#=F)
When is Cb(flat)? In the keys of (Gb,Cb).
When is C#(sharp)? In the keys of D,A,E,B,F#,C#.
Gtrdave and big "U", would you agree?
Later,
