People nowadays use the terms vellum and parchment interchangeably to describe medieval writing surfaces but in fact it appears that vellum was from calves and parchment was from goats. After the skins were processed for writing and laid out for a book there was an attempt to make the pages appear uniform so the skin side was always facing the skin side of the opposite page and the same for the non-skin side. In some books you can still see where hair was removed and some scribes would complain about those poorly processed pages and how difficult it made their job of writing.

(The Great Courses – The Medieval World)