We can derive the major key signatures from the circle of fifths.
For key signatures with sharps (move clockwise from C major):
- Order of keys (in ascending fifths): G, D, A, E, B, F♯, C♯
- Order of new sharps (in ascending fifths): F♯, C♯, G♯, D♯, A♯, E♯, B♯ (Father Charles Goes Down And Ends Battle)
- Usage: the key of G has one sharp (F♯), D has two (F♯, C♯), and so on
- Find tonic: Add a half step to the last sharp in the key signature to get the tonic. If the key signature contains F♯ and C♯, then a half step above C♯ means the tonic is D.
For key signatures with flats (move counterclockwise from C major):
- Order of keys (in descending fifths): F, B♭, E♭, A♭, D♭, G♭, C♭
- Order of new flats (in descending fifths): B♭, E♭, A♭, D♭, G♭, C♭, F♭
- Usage: same as above
- Find tonic: The tonic is the penultimate flat. If the key signature contains B♭ and E♭, then the tonic is the second-to-last flat, B♭.