The circle of fifths is a visual and conceptual tool used in music theory to understand the relationships between different musical keys and their respective scales. It arranges the 12 major and 12 minor keys in a circular pattern, starting with the key of C major at the top.

The circle of fifths is called so because it moves clockwise, with each key a perfect fifth (or seven half-steps) higher than the previous one. This progression helps musicians and composers determine the number of sharps or flats in a key's signature and identify related keys. For example, if you start with the key of C major (which has no sharps or flats), moving a fifth clockwise takes you to the key of G major (which has one sharp), and moving a fifth counterclockwise takes you to the key of F major (which has one flat).

In addition to key signatures, the circle of fifths is also used to understand chord progressions, modulations, and harmonies in music. It's a valuable tool for musicians and composers to explore the harmonic relationships between different keys and to create musically pleasing compositions.