In the 1950s, a single was as wide as the average person’s shoulders. A twin was a single & a half. A double was two singles. A queen was a single & a twin. A king was 2 twins (or 3 singles). Nowadays, the single is forgotten, so everyone is confused. When I was a child in the 1950’s, my bed was a twin and you could put two first graders in it side by side. My best friend had a single bed, so when I slept over at her house, I slept in her bed and she slept on the floor.
EDIT: I don’t think that that’s correct, though, because while beds have become larger, it sounds like the twin was very much around in the 1950s.
Single beds, also known as twin beds, are the smallest antique bed size and the most common size for children’s bedrooms. They were widely used in the late 19th and early 20th centuries and measured around 39 inches wide and 75 inches long. However, some antique single beds may be shorter, measuring around 72 inches long.
According to this:
https://english.stackexchange.com/questions/27810/why-do-americans-call-single-beds-twin-beds
EDIT: I don’t think that that’s correct, though, because while beds have become larger, it sounds like the twin was very much around in the 1950s.
https://www.ebohemians.com/a-guide-to-antique-bed-sizes-understanding-the-history-and-variations/