Following the clef, the key signature on a staff indicates the key of the piece or song by specifying that certain notes are flat or sharp throughout the piece, unless otherwise indicated with accidentals added before certain notes. When a sharp is placed before a note, this makes that note one semitone higher. When a flat is placed before a note, this makes that note one semitone lower. Double sharps and double flats are less common, but they are used. A double sharp is placed before a note to make it two semitones higher. A double flat is placed before a note to make it two semitones lower. A natural sign placed before a note renders that note in its "natural" form, which means that any sharps or flats applying to that note from the key signature or from accidentals are cancelled. Sometimes a courtesy accidental is used in music where it is not technically required, to remind the musician of what pitch the key signature requires.