I believe this varies from country to country - it's certainly different in the UK and the US as I've know some US tourists get very confused over our room descriptions and end up booking the wrong thing. Here in the UK double bed means one bed that sleeps two people and single bed is a small bed that only sleeps one person. I think you may find that in the US double means 2 beds in the room and single means 1 bed in the room.
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