When a key is flattened, the result is a key that is one semi tone to the left. It is pretty obvious to find D flat, E flat, G flat, A flat or B flat since these are black keys. However, when you flatten F or C, you still have to move one semi tone to the left. The surprising result is not a black key, but a white one. This is the reason that: F flat is E. C flat is B. The opposite thing happens when you use the sharp sign: you move one semi tone to the right. C sharp, D sharp, F sharp, G sharp and A sharp- are all black keys, but B sharp and E sharp are white ones: B sharp is C and E sharp is F.