In music theory, the circle of fifths (or circle of fourths) is a visual representation of the relationships among the 12 tones of the chromatic scale, their corresponding key signatures, and the associated major and minor keys. More specifically, it is a geometrical representation of relationships among the 12 pitch classes of the chromatic scale in pitch class space. One of the best sites devoted to the Circle of 5ths is All Guitar Chords. One can easily determine a chord progression based on one or more circles (for example Key of G and Key of C). As noted here, many pop songs change keys during the bridge section. Some chords are even saved for this form development.