The cajon is, basically, a box that is used like a drum.  It's played by sitting on top of the box and playing the front face between the legs.

It originated in South America (possibly Peru) but is now an essential element of flamenco.  If you're looking for a cajon to accompany flamenco, make sure it's the Spanish version - it has strings or metal objects added to make it resonate. Some South American cajones are just a hollow box.

If you're a dancer, it's worth having some cajon lessons because it gives you a different insight on the rhythms.

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Photo by Arteunporro