What are double sharps/flats for?

[leigh anne]

Hello! Lots of times I have encountered the double sharp and flats in tons of pieces. When I see them I know what to do (basically it is just self explanatory based on the name itself you just raise or lower your notes a whole step) and I do it but that always left me wondering why they had to create that symbol and put it before a note, let's say A, and then you play B because it is double sharp. But why do they use the double sharp rather than write the B note itself? Please reply if you know the answer to this question. Thanks!

- Leigh

[brogers70]

Hello! Lots of times I have encountered the double sharp and flats in tons of pieces. When I see them I know what to do (basically it is just self explanatory based on the name itself you just raise or lower your notes a whole step) and I do it but that always left me wondering why they had to create that symbol and put it before a note, let's say A, and then you play B because it is double sharp. But why do they use the double sharp rather than write the B note itself? Please reply if you know the answer to this question. Thanks!

- Leigh

You use double accidentals to make the harmony more clear. For example, suppose you are writing a piece in d# minor and at some point you introduce a D# Major chord - that would not be unusual. It would be jarring to write that chord as an Eb Major chord, and it would be even weirder to write it out as D#-G natural-A#. The least obnoxious way to spell that chord in that situation would be using an F double sharp. Notation is not solely meant to show you where to put your fingers, but also to help you understand the harmony.

[leigh anne]

Thanks brogers70!
 

[lelle]

brogers70 said it all, it's for music theory reasons. Composers who wrote classical music constructed music using certain rules or basic building blocks. The double sharps and flats are needed in some cases to highlight those building blocks properly. This makes how the composer thought more clear both to the composer as they write the music and to the performer. When you play at a sufficiently advanced level, understanding how the music was constructed is essential and helps you decide how you will play a certain passage, where to put emphasis, where to create an extra sensation of tension, where to create a sensation of resolution and relaxation, and so on. The way the music is spelled (double sharps versus a natural note and so on, brogers70 gave some examples of this) is essential for communicating this.

[klavieronin]

Also keep in mind that music notation wasn’t designed for the keyboard. It was originally developed for singers and at a time before equal temperament, when an Eb (for example) was not the same thing as a D#.
For singers it’s usually much easier to think in terms of intervals than absolute pitch and so recognising a D# to F## as a major third is easier than trying to figure out how to sing a diminished fourth.

[quantum]

The reasons behind double sharps/flats are clearer when we transpose into different keys.  Think of a scale or passage of music as being a recipe, when we transpose we want the transposed scale or passage of music to be representative of the originating item - to use the same recipe. 

If we take a recipe of banana cake designed for 10 servings and want  to transpose it to 50 servings we would want it to still look and taste like banana cake, not be weirdly morphed into broccoli soup because we messed with the ingredients as the result of not following guidelines in transposition.

A scale can be represented by letter names.  Take C melodic minor for example:
Ascending
C, D, Eb, F, G, A, B, C
Descending
C, Bb, Ab, G, F, Eb, D, C

The pattern of steps can be described as (T=tone or whole tone, S=semitone)
Ascending:
T, S, T, T, T, T, S
Descending:
T, T, S, T, T, S, T

For the sake of the explanation let's call these sharps/flats, naturals, double sharps/flats theoretical jargon terms.  We will use something more generic like numbers, and refer to these numbers as scale degrees.
Ascending
1, 2, 3 (lowered by 1/2 step), 4, 5, 6, 7, 8.
Descending
8, 7 (lowered), 6 (lowered), 5, 4, 3 (lowered), 2, 1

These numbers or scale degrees form our recipe for the melodic minor scale, and this works in all keys, no need to change the recipe to apply it to a different key.

Now let us transpose this C melodic minor scale into G-sharp melodic minor.

If we were to use convenient letter names and accidentals we might come up with:
Ascending:
G#, A#, B, C#, Eb, F, G, G#
Descending:
G#, F#, E, Eb, C#, B, A#, G#

But something is wrong, the spelling does not look right, it does not spell like a melodic minor scale.  The recipe has been skewed and does not match the recipe with numeric scale degrees outlined earlier.  Some letter names have been used twice, while others are missing altogether.

Following the recipe of scale degrees, the correct spelling for G-sharp melodic minor is:
Ascending:
G#, A#, B, C#, D#, E#, Fx, G#
Descending:
G#, F#, E, D#, C#, B, A#, G#

Notice the consistent incremental letter names regardless of accidental.  Also notice that an F Double Sharp was required to stick to the recipe, to ensure both the pitches and note spelling were transposed correctly.  We also needed to use the spelling of E-sharp for a white key on the piano to remain consistent with the pattern.

When we transpose, we need to make sure all of the following are accounted for: the pitches have to be proportionally shifted to the new key, the spelling of notes must also be proportionally shifted to the new key, the pattern of steps must remain the same.