An approximate date should always be supplied to the description of an undated book; but this date should not be a mere haphazard conjecture, but should be determined by an examination of the characteristics of the book, and comparison with dated books from the same press, so that the date that is ascribed is merely another expression for the characteristics noticed in the book. It is only after careful study that accurate dates can be ascribed to books of a particular press, and monographs on particular printers must be consulted when it is possible. On the question of sizes there seem to be many opinions. There was originally no doubt on the subject, and there is no reason for any doubt now. There are two opposing elements at work, size and form. Originally, when all paper was handmade, and did not vary very much in measurement, books were spoken of as folio, quarto, octavo, etc., according to the folding of the sheet; and these terms apply to the folding of the sheet. In the present century, when paper is made by machinery, and made to any size, the folding cannot be taken as a criterion, and the various sizes are determined by measurement, the old terms, applicable only to the size by folding, being retained. What has evidently led to all this confusion is the application of the same terms to two different things. In describing old books, the old form size should be used, being the only one which does not vary. Under the other notation, a cut-down copy of a book in quarto becomes an octavo, and thus two editions are made out of one. The size of an old book is very simply recognised by holding up a page to the light. Certain white lines, called wire-marks, will be noticed, occurring, as a rule, about an inch apart, and running at right angles to the fine lines, These wire-lines are perpendicular in a folio, octavo, 32mo, and horizontal in a quarto and 16mo. In a 12mo, as the name implies, the sheet is folded in twelve; and in the earlier part at least of the sixteenth century this was done in such a way that the wire-lines are perpendicular; the height of the sheet forming two pages, as is the case in an octavo, while the width is divided into six, instead of four as in an octavo. The later habit has been to fold the sheet differently, the height of the sheet forming the width of four pages, and the width of the sheet the height of three pages; consequently the wire-lines are horizontal. Among early printed books the 12mo is a very uncommon form; quartos are most numerous, and after them folios.