These quires of the codex of our homo modernus are dedicated to the craft of marking text with symbols rather than with words.
It was fairly common in the early Middle Ages that users annotated books. We may shirk at such a behaviour today as rude, but the truth is that the margins of a medieval codex were often the prime space where readers interacted with the text. Because of this medieval habit, we have many types of marginalia that we can study to understand the medieval mind. This was roughly the objective of the Marginal Scholarship project, which run at the Huygens Institute in The Hague in 2011-2016.
As a part of this project, your humble narrator studied the practice of annotation of medieval books by means of special technical signs – the notae. At least 70 such annotation symbols were current in Late Antiquity (c. 300 – c. 600) and in the early Middle Ages (c. 600 – c. 1000), and these are naturally just two of the historical periods, in which parchment manuscripts were produced and read! The function and meaning of many of these symbols is no longer understood today because they fell out of use already during the Middle Ages or in the early Modern period. It was my task to try and understand how were they used, who used them, whether there were any annotation symbols specific to certain regions, periods or communities, and, if it was possible, what purpose they had.

If you are curious to know all about these notae, you can read my book.
If you want a quick overview of the most commonly used early medieval symbols, check out this handy cheat sheet.
In the future, I hope to add here more about individual notae, their histories, and perhaps even publish the ever-growing database of interesting manuscripts annotated with technical signs.