Outer ring: major keys. Inner ring: their relative minors. Shaded: the six major and minor triads that belong to the selected key.
The chord is shown in close position: the bass note in the chosen octave, each other note placed just above the one below it.
About the circle of fifths
Going clockwise, each key is a perfect fifth above the last and adds one sharp (or removes one flat) from its key signature. Neighbouring keys share six of their seven notes, so moving one step around the circle is the smoothest possible change of key.
For any major key, the chord on the key itself (I), the key to its left (IV) and the key to its right (V) are the three major triads of that key. The three minor keys on the inner ring directly below them are its three minor triads: vi, ii and iii. Only the diminished triad on the seventh degree (vii°) is missing from the circle.
At the bottom of the circle, three pairs of keys sound the same but are written differently (enharmonic keys): B and C♭, F♯ and G♭, D♭ and C♯. Use the switch button beside the key signature to change the spelling.