Consider this: you wouldn’t think so looking at a medieval page, but on average only half of it was filled with the actual text. A shocking fifty to sixty percent was designed to be margin. As inefficient as this may seem, the space came in handy for the reader. As the Middle Ages progressed it became more and more common to resort to the margin for note-taking
Both this history discussed and example pictures are absolutely fascinating. Scribbling in the margins has been around for almost as long as books themselves.