If you were a fancy person in the Middle Ages, you might have known how to read. If you were very, very fancy, you might have owned a book. And if that book was fancy, the pages would have been made of a material called uterine vellum.

Let’s break that phrase down. In this context, vellum refers to a type of thin parchment made from animal skin. Although the word vellum comes from the French word for calf, it could come from any number of animals. Records suggest that people used everything from cows to squirrels.

And uterine? As Gwen Pearson explains in the Washington Post, historians generally believe the term refers to vellum made from the skins of stillborn animals, not actual uteruses. This is more specific, but not any less gross. It's also really impractical, since fetal animals have such tiny little skins. Parchment makers would need huge piles of tiny carcasses to yield the amount of vellum they needed.

So how was this stuff made? And of what?

In today’s technological climate, you would think such a question would be pretty easy to answer. You’d be wrong. To run molecular tests on historical documents, scientists would need to sample the material—that is, cut a piece out. And that sort of behavior doesn’t go over too well with archivists.

Researchers knew there had to be a better way. They found it in a most unexpected place: a desk drawer. Pencil erasers made out of PVC are a favorite tool of conservationists, who use them to remove stains from parchment artifacts. Rubbing the eraser against the document creates a teeny electric charge that draws in dirt and other foreign materials.

The erasers gave University of York biochemist Sarah Fiddyment an idea. If the eraser picked up soil, she thought, it was probably also picking up molecules from the parchment itself. Collecting the little crumbs left over after erasing could be a way of non-invasively obtaining samples.

Fiddyment joined forces with other researchers and archivists around the world to collect molecular samples from hundreds of vellum documents created in the 13th, 17th, and 18th centuries. Analysis of the samples found that most of the vellum came from cows and sheep.

But the animals used varied over time. By figuring out which animal skins were being used, the researchers were able to determine what kinds of livestock were most common in the regions where the parchment originated.

“By shedding light on the different animal species used in the production of books in specific areas and periods, the data given by DNA analyses can provide us with a better knowledge about medieval economy as a whole,” University of Liverpool historian Damien Kempf told the Post

And how about those uteruses? “Because the parchment is very fine, very thin, people assumed it must be from small animals,” Fiddyment said in the Post. “We thought, ‘well, realistically, just how many rabbits are we talking about?’" 

They did the math. Thirteenth-century bookmakers were turning out no more than 200 bibles a year. If each bible had an average of 474 pages, and if uterine vellum was made from stillborn animals, parchment makers would have needed 55,000 rabbits, 27,500 stillborn sheep or goats, or 18,000 stillborn calves. If, instead, they actually used young calves—the kind eaten as veal—they only would have needed 4500. During that time, the researchers found, people were eating about 306 veal calves a week in Paris alone. That would have been more than enough skin to go around. 

Just to be sure, Fiddyment and her colleagues compared the proteins in a stillborn calf, a young calf, and a piece of old uterine vellum. The vellum and the stillborn calf had nothing in common, suggesting that uterine vellum might be nothing more than the veal of the parchment world.

The study results were published in two papers: one in the Proceedings of the National Academy of Sciences, and one in the journal Philosophical Transactions B

Parchment and vellum are still very much in use today. At this very moment, the British government is trying to decide whether or not they will continue writing Acts of Parliament on goat-skin vellum. They’ve been through this before. In 1999, the House of Lords approved measures to discontinue the 500-year-old practice in favor of archival paper, but the idea was smacked down in the House of Commons.

The pro-parchment politicians were adamant about preserving their traditions. Some, like MP Gerald Hogarth, argued it was a matter of pragmatism. "Who is to say whether archival paper will last 300 to 400 years?” he asked the BBC. “We shouldn't take the chance."