What is a relative key?
Circle of Fifths
The circle of fifths is a sequence of pitches or key tonalities, represented as a circle, in which the next pitch is found seven semitones higher than the last. Musicians and composers use the circle of fifths to understand and describe the musical relationships among some selection of those pitches. The circle's design is helpful incomposing and harmonizing melodies, building chords, and modulating to different keys within a composition