Castanets are percussion instrument (idiophone), much used in Moorish, Ottoman, Ancient Roman, Italian, Spanish, Portuguese and Latin American music. The instrument consists of a pair of concave shells joined on one edge by string. These are held in the hand and used to produce clicks for rhythmic accents or a ripping or rattling sound consisting of a rapid series of clicks. They are traditionally made of hardwood, although fibreglass is becoming increasingly popular.   Wikipedia