The thick, polished animal skin used as paper during the Medieval era wasn't very strong. It was really easy to rip during cleaning and preparation. But nobody could afford to toss away a page just because of some holes. So illustrators and scribes created art out of the flaws in their pages.
Medieval book historian Erik Kwakkel found these amazing examples of cleer.
(via Medieval Books)
(via Reading Revolutions)
The images are from Erik Kwakkel's awesome Tumblr blog, except when noted otherwise.