Double Sharps and Double Flats
Sometimes in music, particularly when notating chords it is necessary to use double sharps (indicated by !) and double flats (indicated by ~). The reason for this is the spelling of the intervals. E.g. a B augmented triad would contain the notes B, D# and F!.
You will notice F double sharp is the same as G natural. The reason F double sharp is used is that the interval of a major 3rd above D# must contain the letter F and not the letter G; The same principle applies with double flats. E.g. an Eb diminished triad would contain the notes Eb, Gb and B~. The B double flat would be used instead of A natural because the minor 3rd interval above Gb must contain the letter B and not A. These are more examples of enharmonic notes. You may also have noticed the use of notes such as Cb, Fb, B# and E# in some of the previous examples. Although they are not common, these notes are used in music and are therefore worth learning.
