If you have ever wondered on why a book is the size it is, other than something you can hold easily in your hand then you are not alone. When I first qualified as a lawyer, long before the days of the computer and A4 sized sheets of paper, we used a paper size we called ‘brief size’. The secretaries had typewriters, (remember those ) with elongated carriages to take the paper. When is was folded in half it was roughly the same as the current A4. Instead of typing along the short side, briefs to counsel were typed along the other longer side and then they were folded in four and tied with pink tape. When a solicitor was preparing the bill for a case in which they had instructed a barrister, one of the items on the account was the number of folio’s in the brief and that was arrived at by counting a certain number of words, I think it was 72. Of course, the word is used to define a piece of paper folded in two giving four pages in total. But why is a book clearly made up of many folio’s the physical size it is? The answer lies with the size of a medieval sheep.
Paper was first invented in China, but it was the Egyptians that brought it to the west and they used papyrus to make it, which is where the word paper comes from. There wasn’t any papyrus plants in the UK so they used something medieval Britain did have plenty of – sheepskins. Once the skin has been treated and cut into a rectangle, and then folded in half, the ‘pages’ are the size of a modern atlas and that is called a folio. Folded in half again; this is the size of a modern encyclopaedia and is called a quarto. Another fold gives the size of most hardback books and is an octavo. One more fold would give you the size of a mass market paperback. But it does depend on the size of your sheep.
I am grateful to Mark Forsyth for his explanation in his wonderful book The Etymologicon of how and why books are the size they are and where the words to describe them came from.