accidentals
Signs used to change the pitch of a note.
key signature 
Collective term for the accidentals placed after the clef to indicate the tonality of a composition.
sharp 
Accidental placed before a note that raises it by a semitone.
natural 
Accidental that cancels the effect of another accidental.
double flat 
Accidental placed before a note that lowers it by two semitones.
double sharp 
Accidental placed before a note that raises it by two semitones.
flat 
Accidental placed before a note that lowers it by a semitone.