The circle of fifths is a visual representation of the relationships between the twelve tones of the chromatic scale. It arranges these tones in a circular format, where each note is a perfect fifth apart from its neighbors. This tool helps musicians understand key signatures, chord progressions, and the structure of music. In the circle, moving clockwise from any note adds one sharp to the key signature, while moving counterclockwise adds one flat. For example, starting from C major, the next key is G major, which has one sharp, followed by D major with two sharps, and so on.