Contains minor spoilers for “Dragonstone,” the Season 7 premiere of Game of Thrones

In Sunday night’s season premiere, Game of Thrones continued its long streak of drawing on the real past to make Westeros come alive by using Samwell Tarly to make a point about real medieval libraries — sort of.

As Sam gets to know the ropes of the Citadel, part of his thankless job is to work at the library, which offers viewers a chance to get a good look at the grandest library in the Thrones universe. One of the details in that set is that the bookshelves, even outside of the restricted section, come with chains.

That detail is one that medieval-history buffs will recognize as true. Such chains were a common sight in early libraries, for a reason that Sam handily illustrates when he makes off with books that are clearly not intended for public circulation. In 1931, the Oxford scholar B.H. Streeter published a study of England’s early chained libraries, he explained where the tradition came from:

In the Middle Ages books were rare, and so was honesty. A book, it was said, was worth as much as a farm; unlike a farm, it was portable property that could easily be purloined. Valuables in all ages require protection. Books, therefore, were kept under lock and key. This was done in two ways: they were either shut up in a cupboard (almery or armarium) or a chest, or they were chained, sometimes four or five together, to a desk…

Over time, those desks and lecterns were often replaced with early book shelves, known as presses, similar to the ones seen on Thrones. The most famous still-standing chained library is at Hereford Cathedral, where about 1,500 books are still kept.

Get our History Newsletter. Put today's news in context and see highlights from the archives. Please enter a valid email address. Sign Up Now Check the box if you do not wish to receive promotional offers via email from TIME. You can unsubscribe at any time. By signing up you are agreeing to our Terms of Use and Privacy Policy . This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply. Thank you! For your security, we've sent a confirmation email to the address you entered. Click the link to confirm your subscription and begin receiving our newsletters. If you don't get the confirmation within 10 minutes, please check your spam folder.

However, here those real chained libraries diverge from their fictional counterpart, as the whole point of the chains was that the books stayed attached to their shelves. As Streeter explained, the fact of the chain determined much of the architecture of early libraries. If books could only be moved a limited distance from the shelves or lecterns to which they were shackled, the reading of the books had to take place right there.

And as Henry Petroski writes in his 2010 history of books and their shelves, when the books were removed from the shelves, it was a major hassle. The chains worked by connecting the books to a rod, to which several books could be attached at once. So, if the book had to go elsewhere, “the fixture holding the iron rod in place had to be unsecured and all the chain rings removed until the one associated with the desired book was reached,” he writes. “The book could then have been taken from the lectern, and all the other chain rings would have had to be replaced on the rod in their original order, lest there be tangled chains and confusion about a book’s location.”

As a result, the kind of reshelving work that Sam does on Game of Thrones would have been unnecessary, since the books never went far from their homes.

“That was a bit of disconnect for me,” says Robert Rouse, who teaches medieval literature at the University of British Columbia. “The whole point is that you keep [the book] there.”

MORE: The Genius Innovation That Made the Great Library of Alexandria Work

But, says Rouse, a more significant aspect of the Citadel scenes was spot on when it comes to the history of chained libraries. Though the practical purpose of the chains is clear — they keep people like Sam from running off with valuable books — they also serve a deeper purpose.

This ideological purpose, in Rouse’s terms, is to underline the fact that the content of the books, not just the written objects, is also valuable. In a world like the real Middle Ages or the fictional Westeros, with low literacy rates and scarce written material, the person who controls the knowledge holds a seat of power. Even within the hierarchy of maesters, delineated by chains and keys, knowledge is held as a secret to which only a privileged few have access.

“I think it’s kind of interesting if you think about the role of the maesters; the maesters themselves are all about the control of knowledge, and the books are part of that,” Rouse says. “That’s why they’re valuable to society so they guard that very jealously.”

The chained book, as he sees it, is a metaphor — and one that doesn’t require any medieval-studies expertise to understand.

“We can see a lot of that in terms of today, everything from who can access certain websites in China to Wikileaks,” he says. “Information is power, and when you hold information you control the narrative.”

Write to Lily Rothman at lily.rothman@time.com.