Castanets - A Brief History:
Castanets are a member of a very old family of musical instruments that has been found on every civilized continent, with some examples dating back 10,000 years. The "modern" style of castanets probably originated with the Phoenicians, who passed it on to the Iberians, who called them "crusmata." Their descendants evolved the instrument and have kept it in use continuously for the last 2500 years or so.
The Spanish word for castanets is castanuelas, derived from castana, meaning "chestnut" or "hazel" -- castanets were traditionally carved from these woods. The Andalusian word for castanets is "palillos."
So What are Castanets, Exactly?:
The modern castanet comprises a pair of shell-shaped flattened wooden clackers which are held together with a single loop of string or thin leather. The leather is doubled and the thumb is placed through it, and the pair of castanets then hangs freely from the thumb and is manipulated by the fingers and the palms. Accomplished castanet players can make a variety of noises with the castanets, from a flat "click" to a warm roll. Castanets are always played in pairs, and each pair is tuned differently. The higher-pitched pair (known as "hembra," or "female") is traditionally held in the right hand and the lower-pitched pair (known as "macho," or "male") is traditionally held in the left hand.
Castanets in Folkloric Dancing:
Though many people associate the castanets with flamenco, they are not a traditional element of flamenco music or dance; rather, the castanets are an integral part of folkloric Spanish dances, primarily Sevillanas and Escuela Bolera dance.
La Argentina and the Modern Castanet Style:
Antonia Mercé y Luque (1890-1936), known as La Argentina, was a classically-trained ballet dancer who decided to leave ballet and explore Spanish traditional dance instead. Essentially reinventing the entire genre, she brought Spanish folkloric dancing to the stage and rebirthed it as a fine art. She was, by all accounts, an astonishing castanet player, and her style of playing became the definitive one. It's no stretch to say that every modern castanet player is basing their style (however many generations removed) on that of La Argentina.
Castanets in Composed Music:
Various baroque and classical composers have used castanets in their scores, though in modern orchestras, castanets that are mounted on a stick are generally used to perform these pieces. Jean-Baptiste Lully used them in many baroque dance pieces, usually to evoke a Spanish or Arabic feel, and they've been used similarly in many other composed works: Georges Bizet's Carmen, Strauss's Salome, Ravel's Rhapsodie Espagnole, Chabrier's Espana, and Massenet's Le Cid.
How to Play Castanets: The Basics (YouTube)
A Performance by Carmen de Vicente, a Castinet Virtuosa (YouTube)
A Short Traditional Sevillanas Performance with Castanets (DailyMotion)
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Note: buying used castanets, provided they fit your hand well (strings can be changed, of course), is an excellent idea. As is the case with most wooden instruments, the sound of castanets improves with time and lots of playing. The best castanet maker of the last century is said to be Victor Galiano, who has retired but whose castanets can still occasionally be found for purchase at online stores and on online auction sites. Keep your eyes peeled!