The circle of fifths arranges keys by perfect fifths: to the right each key adds a sharp (C, G, D, A, E, B, F♯) and to the left it adds a flat (C, F, B♭, E♭, A♭, D♭, G♭). Their relative minors are inside.
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It helps you know the key signature of each key, find neighboring keys to modulate to and understand progressions that move by fifths, like the II–V–I.