Many medieval books were a joy to look at even when they were closed. Various shiny "add-ons" were drilled in and attached to the wooden boards
on the outside of the book. The most pronounced of these are the so-called "bosses", protective metal pieces attached to each corner of the binding. Much more common are clasps, pieces of metal that kept the book closed.
These were needed because, unlike paper, parchment has a tendency to expand, which could push the book open. A clasp was therefore needed to keep the book closed when not in use, protecting the text inside. Also frequently added to the binding is decoration - flower motifs, playful line patterns, and at times even a painted scene. Such decorative elements on the outside of the binding became particularly common near the end of the Middle Ages.