The internet source on Castanets, listed below, tell us that Castanets are part of Spanish folklore and culture.
They are part of the cultural history of Spain.
Originally castanets were made of wood but with the passage of time, they have also been made of fibre glass and pressed cloth.
They were used as far back as the middle ages in Spain but their origin according to the flamenco research article online, goes back to the Phoenicians.
They called them rattle snakes because of the noise they make!
Flamenco castanets are round and hollow, generally made of wood.
They are not identical, the one with the higher producing sound being called the female castanet which is held in the right hand and the one with the lower sound which marks the time is called the macho.
The castanets are used to enhance the drama of the artist.