Hello! I’m going to do something non-standard for AskHistorians and not cite my sources :::gasp::: But my reason for that is my paleography teacher (the guy who taught me how to read old manuscripts) was eccentric and taught out of his own photocopies, rather than assigning a textbook, so I don’t have a published book to cite. Instead I’m citing my notes and my (limited) experience working with manuscripts.

The very short answer to your question is that the Medieval eraser was actually a knife. How did that work, you ask? Medieval manuscripts were written on animal skins, parchment (usually sheepskin) or vellum (specifically calfskin) and so they were significantly sturdier than the types of paper we use today. If a monk made a mistake, he could simply scrape off the ink from the top layer of the skin and rewrite whatever he needed to. Scribes kept sharp knives with their writing supplies for this purpose, as well as to do other tasks such as sharpening their pens. The downside to this method was that the monk was scraping off part of the parchment with the ink, which could weaken the parchment; monks sometimes even accidentally scraped straight through the page.

When scraping wasn’t possible for whatever reason, crossing out the mistake and writing in the correct letters or words was also done.

Now a word on the images you’ve seen of monks creating seemingly seemingly perfect manuscripts. Monasteries in the Middle Ages maintained something called a scriptorium. Basically a group of monks who created books. Most of the really nice medieval manuscripts you see reproductions/photos of were created by these monks, sometimes for use in the monastery, sometimes for nobles who commissioned a book (in the later Middle Ages this began to move outside the monastery to bookmakers/binders as well). This type of book was usually a Bible or a devotional book like a psalter or book of hours, which was intended to be used to direct the reader in prayer. These were often show-pieces, and they are where a lot of beautiful medieval art is found. A lot of care went into making them, and mistakes were usually corrected.

However, that type of book is usually not the type of book historians get to work with. There were many books which were written or copied in the Middle Ages that had misspellings and bad handwriting, and the scribe (or the author) wouldn’t necessarily go back to correct those mistakes. Sometimes they also include doodles and notes along the side, which are called marginalia. These books would be things like copies of non-religious texts on philosophy/science, theological works, history, etc. which were being copied or even newly written for working use. The scribe might choose the crossing out method if he noticed a mistake, or just leave it. There were also a lot of other written documents (like court records, tax records, personal letters, etc.) which also have lots of crossouts and doodles and crappy handwriting and abbreviations and food stains and everything else you would expect free a world where everything written was written by hand.

There were also plenty of active authors in the Middle Ages, and occasionally we have their original work with their original handwriting (technically called their “autograph” meaning here not their signature but their own personal handwriting style). You can, for example, look up examples of Thomas Aquinas’s actual handwriting, and see for yourself some of the worst handwriting of all time.

They often look pretty exquisite! The ones we see the most are the fanciest ones…thinking of the Book of Kells for example, or the Tres Riches Heures du Duc de Berry. But not all medieval handwriting looks that luxurious. The richly illustrated ones with big, clear handwriting took a long time to produce and cost a lot of money. Books that were “mass produced” (as far as anything could be mass produced in the Middle Ages), or books that didn’t have to be fancy and decorated for a rich client, sometimes have mistakes, and copyists came up with various ways to correct their errors.

One way to do this is by scraping off the ink that’s already on the page. This can be done for an entire page or even an entire book, but it can also be done for individual words. A copyist would use a knife to scrape off the ink. Medieval depictions of monks writing in a book always show their pen, their ink, and just as importantly, their knife.

If it’s an old book and the dry ink is scraped off, we can probably still see the original writing underneath, even today. A famous example of this is the Archimedes Palimpsest, a medieval religious text written overtop of an older mathematical text. But a palimpsest is really any text that has been overwritten, even if it’s just one word.

Other ways to delete a word or part of a text wear to surround it with dots, or like we might do today, simply scratch over it with ink or draw a line through it. The correct word/phrase could be written beside it (if the mistake was caught right away), or above or below the line, or in the margins.

Mistakes weren’t always caught, and when manuscripts are edited for publication today, the editors will make a note of it (in the “apparatus”, all the notes below the edited text). Sometimes a word or sentence is accidentally written twice. Sometimes the mistake is that the copyist looked too far ahead in the book they were copying, so they skipped a few words or more (a “homoioteleuton”). For example, if a scribe was copying this current paragraph, they might accidentally skip the second sentence entirely, since both the second and sentences start with the word “sometimes”. (A modern editor might be able to compare with another manuscript of the same text to find out what’s missing, but not always!)

Sources:

When learning about this in school we used Albert Derolez, The Palaeography of Gothic Manuscript Books: From the Twelfth to the Early Sixteenth Century (Cambridge University Press, 2003) and Bernhard Bischoff, Latin Palaeography: Antiquity and the Middle Ages, trans. Daibhm O’Cróinin and David Ganz (Cambridge University Press, 1990). Some of it is also a bit anecdotal though, just from personal experience reading and trying to edit manuscripts.

Edit: Ack, I left the page open too long and didn't notice u/butter_milk's excellent answer! Hopefully this one will also be helpful.