In the early Middle Ages, books were made from animal hides known as parchment, rather than from paper. Preparing the parchment was a delicate business. The freshly skinned hide is first washed to remove blood and grime, and then soaked in a strong alkali solution to loosen out the hairs. After staying in the de-hairing solution for more than a week, the skin is attached to a wooden frame and stretched tight like a drum. While the skin is drying, the parchment maker would take a sharp knife and scrap the skin to remove the last of the hair and get the skin to the right thickness. This was the most delicate part. Too much pressure during the scrapping process or a slip of the knife could leave elongated rips or holes on the parchment.
Rips were common, so instead of throwing away the damaged parchment, the holes and tears were repaired by stitching, sometimes very artistically with silk of various colors as in embroidery. These pages (pictured above)are from a 14th-century book. The holes in them were mended with colored silk threads sometime after it was purchased by the monastic library at Vadstena Convent in 1417. Repairing holes this way was a common practice, especially in some monastic communities.
Sometimes, depending on the attitude of scribes who did not mind holes on the page, rips were left unrepaired, as you can see in the image above. The hole is perfectly aligned over a drawing of a dragon on the next page.
At other times, holes are cleverly incorporated into art. This image shows a page with three holes that was turned into the face of a laughing man.