Double sharp, double flat What's the purpose?

I've known about double flat and double sharp for a while and I just have one question...

What the heck is it's purpose? I mean, If it's F G A## why make it complicated and make it double sharp when it could simply F G B??? I don't get it.

Can someone please clear this up for me.

Hi =) Its not that complicated, and its actually very usefull

It so much easyer to look at the sheets when this is used correct.
Ill fix an image of it, and youll tell me what is best to look at



In both pics they mean the same, but I would rather relate to the upper picture, cause its less to have to focus on. Its not that hard at all Its just another little thing to llearn, and I feel its there for a reason

Basically they exist because when you write a diatonic scale, you must use every single note, but just only once.

One example:
The C major scale goes like this: C D E F G A B
The G major scale goes: G A B C D E F#
and so on...
Ok, so what happens if you need to write the G# major scale? what do you do? (of course you'd use Ab major, but that's not the point here )
You could write this: G# A# B# C# D# E# G
But then you have 2 Gs and no F, and you don't want to leave the poor F out, do you? So you do this: G# A# B# C# D# E# F##
And there you go

Another example (and this is better than the previous one, I guess): on Prelude #4 by Chopin (sheet music here http://www.mutopiaproject.org/cgibin/piece-info.cgi?id=921) on the 16th bar there's a gruppetto (a "turn") ahead of an A#. This gruppetto would mean you'd have to play the root note, then the upper note, root again, the lower note, then root again. This would become A# B A# G A#. But there's more, under the gruppetto there's a double sharp sign, and when there's a sign under the gruppetto you apply it to the lower note, so you play: A# B A# G## A# (which is enharmonic to A# B A# A A#)
As you may have noticed, there's no other way to write this gruppetto without using the double sharp (well, there is, if you write all the notes on the score, but you can see the double sharp makes it so much clearer)

Well, that's what I can remember about it now. And of course, everything above applies to the double flat aswell

Note: All of this, of course, assuming 12-tone equal temperament, where for instance C# and Db are indeed the same pitch (there are other temperaments where this is not true)

Oh I think I get it! Oh well, I guess I'll just have to learn it

I have never seen the dubblesharp(or flat) been used in any key though. I have only seen it being used as in the pic I posted above.

So even if dubblesharps arent in the keysignature, it can be used in the score to ease the view, as far as I know

Great info from warmal.. But isnt it just okay to write like this: G# A# B# C# D# E# G ? - I dont feel sorry for the F -

Thomandy wrote:I have never seen the dubblesharp(or flat) been used in any key though. I have only seen it being used as in the pic I posted above.

So even if dubblesharps arent in the keysignature, it can be used in the score to ease the view, as far as I know

Great info from warmal.. But isnt it just okay to write like this: G# A# B# C# D# E# G ? - I dont feel sorry for the F -

Ohh poor F, don't say that! If you play in Gb major the F will cry! (hope someone gets this joke )
Anyway, it's only theoretical, Thomandy, you'll never have to use G# major as you can use Ab major instead, and that's only 4 flats.
But to explain why you can't write G# A# B# C# D# E# G, think about the C# major scale. That's an useful scale (and by useful I mean "not theoretical"), and it's written like this: C# D# E# F# G# A# B#. Now, why isn't it written C# D# F F# G# A# C ? I tell you why: because you can't write it on the key signature on the sheet music. See, if you put a sharp in the place of the C (first degree of the scale) on the key signature, how would you write the 7th degree of the scale, the natural C? You can't, the C is already sharp. That's why you use the B#. Same for the F / E#.
And the same principle applies to the F## on the theoretical G# major scale: you already have the sharp on the G, so you can't use a natural G on the key signature, so you must use the F##.
Simple as that

wamaral wrote:

Thomandy wrote:I have never seen the dubblesharp(or flat) been used in any key though. I have only seen it being used as in the pic I posted above.

So even if dubblesharps arent in the keysignature, it can be used in the score to ease the view, as far as I know

Great info from warmal.. But isnt it just okay to write like this: G# A# B# C# D# E# G ? - I dont feel sorry for the F -

Ohh poor F, don't say that! If you play in Gb major the F will cry! (hope someone gets this joke )
Anyway, it's only theoretical, Thomandy, you'll never have to use G# major as you can use Ab major instead, and that's only 4 flats.
But to explain why you can't write G# A# B# C# D# E# G, think about the C# major scale. That's an useful scale (and by useful I mean "not theoretical"), and it's written like this: C# D# E# F# G# A# B#. Now, why isn't it written C# D# F F# G# A# C ? I tell you why: because you can't write it on the key signature on the sheet music. See, if you put a sharp in the place of the C (first degree of the scale) on the key signature, how would you write the 7th degree of the scale, the natural C? You can't, the C is already sharp. That's why you use the B#. Same for the F / E#.
And the same principle applies to the F## on the theoretical G# major scale: you already have the sharp on the G, so you can't use a natural G on the key signature, so you must use the F##.
Simple as that

Awesome I never though about it that way, cause it was just natural to use the enharmonic keys instead
But thats Great to have a face to put on the theory, not just knowing that it is like that, but also know why

Another reason for using a double accidentals is the dominant chord in minor scales with many sharps: for example if you are in G# minor and play a dominant chord, it will be D#7, i.e. D# F## A# C#, and not Eb7 which wouldnt make sense and would be very hard to read.

When I read all these posts I feel like a complete idiot trying to learn to play piano but not knowing at all what the heck you guys are talking about!!! =)
Its like a complete defferent language to me!
I never even heard there was a such thing as a double sharp! =)

Hehe, you don't see double accidentals as long as you don't play pieces in rare time signatures such as C# minor or Bb minor...