Original: Double-flats and double-sharps are used in music as accidentals , but they are never placed in the key signature ( in music that uses equal temperament ) , due to notational convention and because reading the music would become unnecessarily difficult .
Simplified: Double-flats and double-sharps are used in music as accidentals , but they are never placed in the key signature ( in music that uses equal temperament ) , because it would make reading the key signature very difficult .