In Western musical notation, a key signature is a set of sharp, flat, or rarely, natural symbols placed on the staff at the beginning of a section of music. The initial key signature in a piece is placed immediately after the clef at the beginning of the first line. If the piece contains a section in a different key, the new key signature is placed at the beginning of that section.

Musical notation is any system used to visually represent auditorily perceived music, played with instruments or sung by the human voice through the use of symbols, including notation for durations of absence of sound such as rests. For this reason, the act of deciphering or reading a piece using musical notation, is known as "reading music".