C major
The major key with no sharps or flats; relative major of A minor.
What it means
C major is the major key with no key signature: its scale consists solely of the natural notes C, D, E, F, G, A, and B. Its tonic is C, and its relative minor is A minor. On the piano it corresponds exactly to the white keys, making it the simplest key to notate.
Musical use
Because it requires no sharps or flats, C major often serves as a starting point and reference when learning music theory and piano playing: on the circle of fifths it sits at the top, from which sharp and flat keys unfold in either direction. In compositional history, this simplicity has often led to C major being associated with a particularly clear, "innocent," or festive character, as in Beethoven's First Symphony (1800) or the finale of his Fifth Symphony (1808).
The key's lack of accidentals should be distinguished from its actual sound: on an equally tempered instrument, C major is in no way "purer" or differently tuned than other major keys; its special status is purely a matter of notation.