Castanets are a percussion instrument (idiophone), used in Kalo, Moorish, Ottoman, ancient Roman, Italian, Spanish, Sephardic, Swiss, and Portuguese music. The instrument consists of a pair of concave shells joined on one edge by a string. They 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 (chestnut; Spanish: castaño), although fibreglass is becoming increasingly popular.