In the late 19th and early 20th centuries, several outbreaks of infectious disease in the U.S., including the flu, tuberculosis, and pneumonia, prompted public-health officials and homemaking experts to suggest a tweak to American bedrooms as a safety measure: Couples (and children) sleep in separate beds. As a result, twin beds quickly entrenched themselves as a staple in American homes and remained popular long after the plague outbreak was over. In the mid-20th century it was still relatively rare to see depictions of married couples sleeping in the same bed (see: I Love LucyThe Dick Van Dyke Show).

Hilary Hinds, an English professor at the U.K.’s Lancaster University who wrote A Cultural History of Twin Beds, argues that while the smaller beds were a huge cultural fad, it’s not clear numbers-wise exactly how many households adopted them. She cites one 1950 study in her book that found more than two in three beds purchased in the U.S. at the time were twins, compared with just a quarter before World War II (though she suspects that statistic might be a little overblown).

Eventually, twin beds’ popularity among American adults more or less faded into obsolescence, Hinds writes; today, queens are the most popular mattress size purchased, according to Consumer Reports. Kid-size beds have since been mostly consigned to kids—minus one notable exception: college dorms. Of course, the college-dorm mattress is no standard twin, but what’s known as the twin extra-long (XL), five inches longer than its traditional counterpart. The twin XL has a twin-size width—helpful given dorm rooms’ limited space—and its king-size length, presumably, is a bone thrown to the comfort of students whose bodies are, after all, adult-size.

Twin XLs are so specific to colleges that some families begin the college-preparation process having never heard of the size. Jessica Stellmach, a customer-service manager at Bare Home, a company whose subsidiaries include the dorm-products vendor, said her business constantly receives calls from befuddled parents who don’t understand why they need to buy a weird size of sheets.

Colleges have tended to have small beds in dorms for pretty much as long as dorms have existed. A century or so ago, around the same time public-health officials spurred the twin-bed fad, colleges were building on-campus residence halls in an effort to counter the self-segregation of rich students into fraternity and sorority houses and their lower-income peers into low-rent off-campus housing. Archival images of some of the earliest dorms show pint-sizelumpy-lookingcot-like beds tucked into the corners of pint-size, clumsily (if earnestly) decorated rooms.

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