Traditionally they are made from hardwood, although there are cheaper fibreglass models.A player usually uses two pairs of castanets - one pair is held in each hand. A string or cord is hooked over the thumb and the castanet rests on the palm with the fingers bent over to support the other side. Each pair of castanets has a slightly different pitch. The "hembra" (female) castanet is usually held in the right hand (if you are right handed) with the larger macho (male) pair held in the left hand.
The castanets can also be used within an orchestra. Here they are sometimes attached to a handle, or mounted on a base to form a pair of machine castanets. Rolls on the castanets can be produced in a number of ways. When hand-held they are bounced against the fingers and palm of the hand, on sticks by bouncing them between fingers and the players thigh. Machine castanets produce a less satisfactory roll by the player doing a fast alternation of the two castanets with the fingers.
During the baroque period castanets were widely featured in dances. Castanets were used to lend a Spanish atmosphere in George Bizet's opera "Carmen" and Emmanuel Chabrier's orchestral work "Espana".
There are two concerto's for castanets. Leonardo Balada's Concertino for Castanets and Orchestra Three Anecdotes (1977). The other is Conciertino fuer Kastagnetten und Orchester by Helmut M. Timpelan.
Castanets are a seemingly simple but very versatile instruments.