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Double sharp Raises the pitch of a note by two chromatic semitones. Usually used when the note to modify is already sharpened by the key signature

If I write a D note in a D Major key and I want to use a double sharp, why not write an E note instead?

I am confused about why to use double sharps at all.

marked as duplicate by guidot, Carl Witthoft, Richard, Dom Feb 9 '18 at 13:57

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    1) D𝄪 and E are not always the same pitch. 2) Sometimes you need a D𝄪 instead of an E to correctly spell something. For example, a G♯+ chord does not contain an E, but a D𝄪 for the fifth. – David Bowling Feb 9 '18 at 7:27
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We're in the key of, say, B major, and we need a B augmented chord. That means the 5th of B needs sharpening. Thus, F# now becomes Fx (F##). Yes, on a lot of (12edo) instruments, it's the same note, same place as G. But note (sic) that there's no G nat. diatonically in key B, and that's not the note that gets changed anyway - it's F#. In any case, writing Gnat. would still incur an extra accidental, so why not do it correctly?

You quote writing an E note in the key of D, instead of Dx. In D, it would be very rare to need to double sharp a D note. Generally, a double sharp is used when an already sharpened note needs to go up a semitone - and still retain its original letter name - so it would make sense when talking about the interval it makes with preceding and succeeding notes.

  • @LoveIsHere - double accidentals generally occur when building augmented chords in sharp keys (double sharps), or diminished chords in flat keys (double flats). There is an exact algorithm for determining where, although I don't know it offhand. – Stinkfoot Feb 9 '18 at 8:34
  • @Tim considering David Bowling's comment under the main quesiton, is it possile that a F## actually isn't the same pitch as a G?hat is, it's not the same frequency, owing to the not-quite-100% spot-on way that Western scales divide an octave up? (eg a 5th isn't a 'true' 5th)? Please forgive my terminology; I'm not too well read with music theory. – user2808054 Feb 9 '18 at 14:28
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    @user2808054 - hence my ' (12edo)', when the octave is equally divided into 12, as in most modern pianos. Yes, Gnat and Fx may well be played slightly differently by, say, string players, however, that factor didn't really crop up from the question, so didn't need addressing here. – Tim Feb 9 '18 at 14:41
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Another reason: simplicity of notation. Say you have a piece in Bb, and somewhere along the way there's an E-natural followed with an E-flat. Rather than write "naturalsign-E ; flat-sign-E", it's much easier for the performer to read "double-sharp-D; E (defaulting to Eb due to key signature)".

ETA

Thanks to Milo (apparently back from the Tollbooth) for a better example -- playing in G# minor to see Fx moving to G#

  • Thanks but lets say that after all that we need to play d natrual again there would be a need to asign a natrual sine again in the d isnt it? – LoveIsHere Feb 9 '18 at 12:46
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    @LoveIsHere yes, that's true. When music gets seriously chromatic, there's gonna be headaches. – Carl Witthoft Feb 9 '18 at 12:52
  • Wouldn't say it's much easier to read. About the same, I'd say. And it may go against what the note actually should be. I've seen too many e.g. C7b9 written with C# as the 'flat 9'. It's wrong, and confusing - to me at least... – Tim Feb 9 '18 at 12:58
  • Carl, I disagree; I'm with @Tim. In the context of a key signature of two flats, an E (natural) would be fairly common. Double-sharps would be bizarre and thus hard to read. – Rosie F Feb 9 '18 at 13:15
  • An Fb moving to Eb might be okay, but Dx moving to Eb would be confusing, given that one would see an upwards motion in the score representing a decrease in pitch. I would more expect, for instance, in G# minor to see Fx moving to G#. – Milo Brandt Feb 9 '18 at 13:22

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