We have learned that a flat lowers a note by a half step and a sharp raises a note by a half step. The flat and sharp allow the pitch produced when a note is played to change by a half step while the letter name of the note remains the same – so, B can become B flat or B sharp.
Occasionally there is a need to retain the same letter name while changing the note by a whole step. The double flat and double sharp allow the pitch that is produced when a note is played to sound a whole step lower while retaining the same letter name.
Double Flat
The double flat is easy to identify – it looks like two flat symbols.
Double Sharp
The double sharp is a new symbol. Why do we not just use two sharp symbols like the double flat? I don’t know. But we don’t – here is the double sharp symbol.
Uses of Double Flat/Sharp
The double flat and double sharp are not often used, but they are useful in some very specific situations to make the notation look less complex. Let’s look at some of them below.
Chromatic Passage Through Whole Step
One case where a double flat/sharp is useful is when a chromatic passage is used and the key signature has two adjacent accidentals, for example the key of Eb major that has Ab and Bb. Using a double flat/sharp allows the chromatic passage to include only 1 accidental instead of two.
Neighbor Tones
A neighbor tone is when you start on one note, move a step (whole or half) away (the “neighbor” note), then return to the original note. A lower neighbor is when the middle note moves lower; an upper neighbor is when the middle note moves higher.
There are specific cases where neighbor tones use double flats/double sharps to make the notation easier to read.
Lower Neighbor
A double sharp is sometimes used to make a half step lower neighbor easier to read.
If there are two adjacent sharps in the key signature that form a whole step, such as G# and F# in these examples, then a half step lower neighbor from the upper note (G#) is a half step higher than the lower note. This can be written as F double sharp, or as G natural.
Using F double sharp in this case has two advantages:
- There is only 1 accidental instead of 2
- The notes convey the downward motion of the lower neighbor using the double sharp because the letter name changes; using G natural, all notes are on the same line and it is not easy to see the direction of movement
Upper Neighbor
Like the double sharp with a lower neighbor, a double flat is sometimes used to make a half step upper neighbor easier to read.
If there are two adjacent flats in a key signature, such as Ab and Bb in these examples, then a half step upper neighbor from the lower note (Ab) is a half step lower than than the upper note. This can be written as B double flat or A natural.
Using B double flat has two advantages:
- There is only 1 accidental instead of 2
- The notes convey the upward motion of the upper neighbor using the double flat because the letter name changes; using A natural, all notes are on the same line and it is not easy to see the direction of movement
These are just some cases where a double flat or double sharp would be used. There are others, but hopefully you can see the reason for using them from these examples.