When my then-husband and I were newlyweds, we went shopping for a bed to replace his that we were sleeping on. The salesperson was quite insistent that we must be mistaken that we were sharing a full mattress, because most people thought that and discovered after measuring that it was actually a queen. You should have seen her face when we came back with 54" x 75" measurements, proving we were definitely two adults sharing a full mattress.

I grew up sleeping on a double bed my mom had inherited from her grandparents, or rather, from one of them. When she first told me as a young adult that they hadn't shared that bed, I was surprised, but she pointed out that (a) they had originally met via correspondence after one of them wrote to a farming magazine for a pen pal, and they turned out not necessarily to have that much in common, though they got along all right and went on to have four boys, and (b) "there wasn't really much birth control in those days, dear." I'd heard for years that the older double beds were ever so slightly smaller than today's full beds that are called by the same name, which explained why my childhood double mattress fit and I can't find a full mattress that does, but from what I can tell that may not be the case. It may just be that the mattresses we used to get happened to squeeze in to the slightly smaller frame and the ones today are more standard.