4

From wiki:

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.

LoveIsHere
  • 1,217
  • 1
  • 10
  • 20
  • 2
    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. –  Feb 09 '18 at 07:27

2 Answers2

7

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.

Tim
  • 183,051
  • 16
  • 181
  • 444
  • @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 09 '18 at 08:34
  • @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 09 '18 at 14:28
  • 1
    @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 09 '18 at 14:41
4

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#

Community
  • 1
Carl Witthoft
  • 14,656
  • 2
  • 18
  • 41
  • 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 09 '18 at 12:46
  • 1
    @LoveIsHere yes, that's true. When music gets seriously chromatic, there's gonna be headaches. – Carl Witthoft Feb 09 '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 09 '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 09 '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 09 '18 at 13:22