Printing papers are generally white, but sometimes they have been used in colour, green, pink, blue or yellow. Such papers are now and then found in Italian, German, and English books, more rarely in French. Silk and satin have both been used for printing on in England from the seventeenth to the nineteenth century. Vellum has also been largely used for special copies of fine printed books. A recent French book of prayers is entirely woven in white and black silk. It looks like a beautifully printed book with monotone borders. [Illustration: FIG. 61.--“Chain lines” thick, and “laid lines” thin, on paper.] An original sheet of paper can of course be made and cut to any size, and the terms folio, quarto, octavo, and duodecimo do not indicate any actual size except in bookbinders’ specifications. The terms only indicate the number of times the original sheet has been folded, and this obviously is a matter which is subject to as much variation as the printers choose. But fortunately such foldings do not vary much, and so we may safely mention those that are most commonly used. There are several ways of finding this out, the most obvious being to count the leaves which follow any one letter in the white line at the bottom of the leaf. This letter is called the “signature.” If there is an A or A 1 at the bottom of the first leaf, and when eight leaves have been turned over a B or B 1 appears, then the book is an octavo, and so on. Another way of determining the same thing is by means of the direction taken by the chain or wire marks all over the paper, and yet another is to be found by studying the position of the watermark. But neither of these tests are conclusive, and often enough there are neither chain marks nor watermarks to be found at all, and the sheets are not always rightly or carefully cut, which brings the watermark all wrong, so the following notes are only what may be expected in normal books. If a piece of paper or page of a book printed before 1750, and possibly in later work, be held up to the light, certain lines may be seen all over it in a lighter tint than the rest of the page. These appear as long thick lines crossed at right angles by short thin ones. The long thicker lines are known as “chain” lines, and the shorter ones “laid” lines, and they are of some value when they exist for helping to determine the “size” of the book. [Illustration: FIG. 62.--Fol.] --Fol. If the original sheet is folded once it is called a folio, and in this case the chain marks are perpendicular and the watermark is in the middle of the first leaf. In a folio there is one fold, two leaves or four pages, and nothing to cut. Most of Caxton’s books are folios although they are quite small. [Illustration: FIG. 63.--4^o.]