Students in their dormitory at Florida's Bethune-Cookman College in 1943Gordon Parks/ Library of Congress / Getty

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 Lucy, The 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 TwinXL.com, 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-size, lumpy-looking, cot-like beds tucked into the corners of pint-size, clumsily (if earnestly) decorated rooms.

As for those additional five inches? A 2002 Wall Street Journal article suggests they emerged as a college-campus cornerstone sometime around the mid-1970s. Observing that average human height had been gradually increasing throughout modern history, residence-life administrators began replacing the normal twin beds with their slightly longer cousins, the Journal reported.

Men tend to reach their peak height by or before age 20, at which point the average man, nowadays, is 69 inches tall. A man sleeping on a normal twin bed, which is a mere 75 inches long (and a teeny 38 inches in width), might find himself having to contort his body to fit it all on the mattress, or to roll over without falling off. Those extra five inches of wiggle room are a subtle but critical improvement—especially for the above-average-height individual. (Some campuses give athletes a reprieve from the twin XL, housing them in special residence halls with larger beds to accommodate their larger bodies.)

The twin-extra-long beds serve a practical purpose, too. In general, “architects are always thinking about ways of getting people out of their bedrooms,” says Carla Yanni, an art-history professor at Rutgers University who authored the new book Living on Campus: An Architectural History of the American Dormitory. This mentality was particularly evident in dorm design, she told me. Back when the country’s very first dorms were being built, “holing yourself up in a sleeping room with the door closed was not ideal from anyone’s point of view,” she says.

So architects dedicated most of the space in a given residence hall to common areas where students could study together and convene around the fireplace, according to Yanni; their bedrooms, on the other hand, were minuscule. “The idea,” Yanni told me, “was clearly to socially engineer the students, so they would not spend most of the time in the sleeping room.” While residence-hall architecture today is much more varied than it was in dorms’ earliest days, bedrooms still tend to be small, Yanni says.

Of course, most college students don’t live in dorms—according to one higher-education scholar’s analysis of federal data, just 16 percent of all undergraduate students lived on campus during the 2015-2016 school year. Still, there’s something democratizing about the fact that pretty much every freshman living on campus has this same experience. They’re all in it together—“it” being a tiny room with a tiny bed.

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