The circle of fifths is a musical tool showing how the keys are related. 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 equivalents, such as C# and Db, both keys and the sharps or flats in each are listed.
Aside from understanding the keys, sharp and flats, the circle of fifths help the musician to understand the basis for typical chord progressions and therefore helps in the creation of melodies.

You should be able to use the Circle of fifths to work out the sharps and flats as shown in the following table, for any key.