key signature

In music, sharps or flats printed at the beginning of every line (or stave) of music after the clef. It tells a player what key the music is in. A key signature without any sharps or flats means the piece is in C major or A minor, or that the music is nontonal.

As shown above, each key signature is shared by two keys: a major key and its relative minor (the relative minor is found three semitones below the major key).

To change a key signature during a piece, all that is needed is to write the new key signature after a double-bar line.