I’m trying to play Toccata by Debussy and the piece is in the key of E. In the sheet music, the F has a double sharp next to it, so that means I would be playing a G sharp. When I looked at the synthesia, it was playing a natural G. I’m not sure if I’m wrong or the synthesia is but I highlighted the section in yellow.enter image description here

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  • Tangentially, the enharmonic notation seems surprising here: surely the D-naturals in the first measure would make more sense as a C-double-sharps, fitting into A-sharp major and resolving into the D-sharp major on the next beat?
    – PLL
    May 18, 2020 at 22:52

4 Answers 4


Accidentals affect the basic note - if you like, the white key on the piano.

So, regardless of the key signature - which permanently changes certain notes (here F, C, G and D♯), the double sharp applies to a basic F note, making it Fx (F♯♯) or F double sharp.Taking it up two semitones, so it looks like a G note on piano.

What it doesn't do is take the F♯ notated in the key signature, and then double sharp that!

  • 8
    This is correct. The double-sharp accidental replaces the sharp in the key-signature -- it is not in addition to the sharp.
    – Rosie F
    May 17, 2020 at 7:04
  • This must have been the wrong premises of OP. key signature plus double sharp = 3 semitones up! Tim's answer is destroying this error most best. May 17, 2020 at 9:28
  • 2
    Music notation is well adapted to support sight reading and when you see a double sharp you only double sharp the F forgeting whatever F was previously. May 17, 2020 at 11:58

" In the sheet music, the F has a double sharp next to it, so that means I would be playing a G sharp."

No. Accidentals aren't additive. We don't tot up the F♯ in the key signature plus the accidental double-sharp to make a triple-sharp. It's just F double-sharp. Played as G.


F double sharp is enharmonic to G, not G#.

So the note you need to play is a G.


A sharp or flat means you raise or lower the note by one half step. Or, as semitone. When a note has a small cross-like sign next to it, like you have there, you do this twice by raising it a whole step, two semitones. F > F# is one, F# > G is another.

Interestingly there is another double sharp there, before the bar line. In order to cancel a double sharp there would need to be a single sharp there (you did once also need a natural sign but this is no longer necessary).

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