Page "Accidental (music)" Paragraph 19

The double accidental with respect to a specific key signature, raises or lowers the notes containing a sharp or flat by a half step.

For example, when in the key of C-sharp minor or E major, F, C, G, and D contain a sharp ; adding a double accidental ( double sharp ) to F for example in this case would only raise the already sharp containing note F one half step or semitone, creating G natural, respectively.

Conversely, if a double sharp were added to any other note not containing a sharp or flat as indicated by the key signature then the note will be raised two semitones or a whole step with respect to the chromatic scale.

For example, in the aforementioned key signature any note that is not F, C, G, and D will be raised by a whole step or two semitones instead of one, so an A double sharp raises the note A to the enharmonic equivalent of B.