You have some very good descriptions of the castanuelas (castanets) however I'd like to add castanets are not traditionaly flamenco. Flamenco is the dance of the gypsies,which has become a permanent part of spanish culture for quite some time now :). However, what is known as classical spanish (think spanish ballet) the castanets are a vital part of the dance and music. In Andalusian folk dance (sevillanas which you could call a courting dance) castanets are regularly played. In other spanish dances (the jota) they are also played but a bit differently. There are many stereotypes of flamenco and spanish dance, it's much more intricate then stomping your feet around and saying "ole!" the feet become a part of the percussion, every sound is very intention and stays on a particular rythm depending depending on the sound. Every flamenco song has a particular count which is always the same for each song. Dancing is not as simple as moving or even keeping rhythm. The dancer has to keep in mind the count, there are certain steps you will move to communicate with the singer or guitarist. There are down beats, you can even dance in counter time. All this while doing something I can only describe as more difficult than rubbing your belly and patting your head at the same time. Your arms, head, feet and legs can be going in different directions while having to understand the intricate music. So please give us flamencos a little more credit for what we do. Dancers are not just dancers, but also musicians :D