Hi:
i know that tonal music is based in the concept of a KEY. The importance is so great that, in classical music, the key is very often mentionned, along with the Opus number, in order to identify the piece !!
However, there are some things that remain pretty obscure to me

Let suppose you have a piece in C major. C major scale consists of just the white piano notes. It is my understanding that notes outside this scale, the so called chromatic notes (is this right?), should be pretty rare in the piece, because, to the ear, they "not belong" to the mood of the piece. However, i see pieces with LOT of accidentals, that sound very good (Fur Elise, for instance, just in its main motif, has an accidental).
Let us return to our C major piece. G major scale has only 1 difference with C major, that is F# instead of F. Why cannot i re-writte the score of the piece, in G major key? after all, those F# accidentals, if any, will become normal notes, and those F notes will now be accidentals...I KNOW THIS IS INCORRECT

But just want to know exactly why...
The only thing that i observe, in almost every piece, is that the very last note is the key note.

Thanks