Why do C major and A minor have no sharps or flats in their key signatures?

In musical notation, the key signature serves to indicate which notes must be consistently raised or lowered from their natural pitches throughout a piece. The keys of C major and A minor have no sharps or flats in their key signatures because their diatonic scales consist entirely of natural notes.12

The C major scale is composed of the notes C-D-E-F-G-A-B-C, which contains no altered intervals. A minor is the relative minor of C major, meaning the two keys share the same key signature and the same set of notes. The A minor scale is simply the C major scale starting on its sixth tone (A), resulting in the sequence A-B-C-D-E-F-G-A. Because these scales utilize only the natural notes found within the C major collection, no accidentals (sharps or flats) are required at the beginning of the staff to define the key.231