As I understand it, a double flat/sharp mark raises the note by exactly a full step.
However, I don't understand why anyone would use this - after all, the exact same note can be represented with a different symbol much more simply, requiring less effort from the player/reader and generally being clearer.
So when does one use a double sharp/flat and why?
A fifth up from B♯ should look like a fifth up. That makes it an F with an accidental, and that accidental needs to be a 𝄪, so we end up with an F𝄪.
if you are in a key that already has a sharp/flat and you want to raise/lower that note it makes sense to have a double sharp/double flat.
example: key of D major: D E F# G A B C# D -- now you want to raise that F# a half step you want to make a Fx (F double sharp). you could call it a G and that is the note you would play on your instrument, as it is en-harmonically the same but it is more precise and perhaps less confusing once you understand it to use the double sharp.
