2012.1.25 cuba b 562
This is the currency that Cubans use. Called moneda nacional, or simply pesos, it was worth 1/24th of a CUC.
I never did get an explanation from a Cuban of the reasoning behind the dual currencies. A friend of mine who visited about 10 years ago said that when she was there, the US dollar was used as tourist currency. We were under the impression that US dollars would be acceptable for tips and informal exchange, but that wasn't our experience - either we took the money to the currency exchange (el cambio), or they did.
The dual currencies have had the effect of creating a strange, inverted economy. Anyone who works in the service industry and comes into contact with tourists makes tip money in CUCs. Since CUCs are so much more valuable than moneda nacional, families often live primarily on the income earned by whichever family member can get a service industry job. As a result, one often finds doctors driving taxicabs, and professors working as tour guides (like our guide, Amircal).