Key signatures tell you which sharps or flats are used throughout a piece. The Circle of Fifths shows how all 12 keys relate to each other.
Key of
C Major
0
Accidentals
Am
Relative Minor
C
Relative Major
Accidentals: None
Scale Notes
Key Signature Rules
Moving clockwise (up a 5th) adds one sharp: F# → C# → G# → D# → A# → E#
Moving counter-clockwise (up a 4th) adds one flat: Bb → Eb → Ab → Db → Gb → Cb
Every major key has a relative minor that shares the same key signature (3 semitones below)