The key signature of C is at the 12 o'clock position on the circle of fifths.
The order of the major sharp key signatures on the circle of fifths (right side) can be remembered by "Goes Down And Ends Battle Father Charles" (G, D, A, E, B, F#, C#).
The order of the major flat key signatures on the circle of fifths (left side) can be remembered by "Father's Battle Ends And Down Goes Charles" (F, Bb, Eb, Ab, Db, Gb, Cb).
To the right of C on the circle of fifths, the key signatures with sharps, in order from 1 to 7 are G, D, A, E, B, F#, and C#. Example: The key of D major has two sharps (2#).
To the left of C on the circle of fifths, the key signatures with flats, in order from 1 to 7 are F, Bb, Eb, Ab, Db, Gb, and Cb. Example: The key of B flat major has two flats (2b).
The relative minor key for each major key is shown inside the circle. The relative minor key of a major key is 2 letters down from the major key (exactly 3 semitones down). The relative major key of a minor key is just 2 letters up from the minor key (exactly 3 semitones up). Examples: The relative minor key of C major is A minor. The relative minor key of C flat major is A flat minor. The relative minor key of C sharp major is A sharp minor.
