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EMT Lesson Four  How to understand keys and key signatures

Cycle of fifths

The ‘cycle of fifths’ allows you to work out the number of sharps or flats in a key.

The number of sharps and flats in each key has already been established by using our scale knowledge. However, another way to calculate key signatures is by using the cycle of fifths.

For example the key of C major has no sharps or flats. A fifth* (seven semitones) up from C is G. The key of G has one sharp (F#). A fifth up from G is D. The key of D has two sharps (F# and C#).

Each time you start a new key a fifth up from the previous key, a sharp is added.

[*Note: See Lesson 3, Page 2, for an explanation of the interval of a '(perfect) fifth'.]

Figure 18. The cycle of fifths (keys with sharps in the key signature).   (Enlarge)

Figure 18. The cycle of fifths (keys with sharps in the key signature).

Each time you start a new key a fifth down from the previous key, you add a flat (b).

For example the key of C major no sharps or flats. A fifth (seven semitones) down from C is F. The key of F has one flat (Bb). A fifths down from F is Bb. The key of Bb has two flats (Bb and Eb), etc.

Next: Cycle of fourths — the more common way to calculate flat key signatures