Simplifying key signatures

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Major scales (and by derivation, major keys) always include notes using one of each of the letter names from A to G.

This is one of the few rules in music that has no exceptions.

One useful application of this rule is to determine whether a key should have a sharps (#s) key signature or flats (bs) key signature.

Example: Key of B major:

1. Write out letter names: B C D E F G A B
2. Apply sharps to space notes according to major scale formula (Tone, Tone, Semitone, Tone, Tone, Tone, Semitone):
   B (T) C# (T) D# (s/t) E (T) F# (T) G# (T) A# (s/t) B
3. No problem! B major has 5 #s

But what if we tried D# major?

1. Write out letter names: D E F G A B C D
2. Apply sharps to space notes according to major scale formula (Tone, Tone, Semitone, Tone, Tone, Tone, Semitone):
   D# (T) E# (T) F## (s/t) G# (T) A# (T) B# (T) C## (s/t) D#
3. We have had to go to extraordinary lengths to preserve the formula by using double sharps. Life is complicated enough without having to try and play something in a key like that!

So we decide to use the enharmonic equivalent key of Eb instead (this is a fancy way of saying that Eb sounds the same as D#)

In the key of Eb

1. Write out letter names: E F G A B C D E
2. Apply flats to space notes according to major scale formula (Tone, Tone, Semitone, Tone, Tone, Tone, Semitone):
   Eb (T) F (T) G (s/t) Ab (T) Bb (T) C (T) D (s/t) Eb
3. We now have a much simpler key signature - 3 flats (bs) which will sound identical to the exotic key of D# major (9 sharps). Of course we can still tell our audience we're playing in D# major!

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