Puerto Rico is a comparatively small island situated in the Eastern Caribbean and one of the few commonwealth states that the United States has, outside the 50 states that make up the US proper. So, while Puerto Rican people are US citizens, they enjoy a high degree of self governance, except in some matters such as defence, foreign affairs, currency, etc. Other than the native Indians, the island was initially settled by the Spaniards and later annexed by the US. Therefore Puerto Rican culture is really diverse with a predominantly Spanish atmosphere and outlook, and that language being as commonly used as English, if not more so.
