The Circle of Fifths
The circle of fifths depicts the relatedness of keys in an easily-readable chart. The circle tells which keys you can easily modulate to, and which keys you cannot. This is a very useful tool for songwriting, and should help you immensely.
The circle of fifths is a musical tool showing the relatedness of keys. It is based on a simple principle: in the clockwise direction, each new key is the dominant (V) of the previous key. Therefore, in the counterclockwise direction, each key is the fourth (IV) of the previous key. The keys on each side of a given key are closely related keys. For example, if you play in the key of G, the keys on each side of G (C and D) are closely related to G. Closely related keys are easy to change to, using only a few simple chords, or often only one.
In this diagram, the outside letters refer to the keys. Inside the circle, the number of sharps or flats appears for each key. When there are two keys which are enharmonic equivalents, such as C# and Db, I have listed both keys and the sharps or flats in each.
As an example of the uses of the circle, we will try this musical passage which modulates (changes) from G to D, a closely related key. Play this passage to see how it sounds and to better understand the musical devices used.