Books reach the bookbinder either in the sheets just as they left the printing press or folded and stitched. This folding and stitching is, of course, part of the binder's work, so we will begin with the sheet as it left the press; this sheet must in the first place be folded ready for further manipulations.
By folding we mean the arranging of the parts of a sheet in the order of succession required for reading. To facilitate this work the printer marks not only the page numbers on each sheet but also gives each sheet a number, these numbers being known as signatures. Every sheet is printed on both sides; the front side is called the first side, the other side the perfecting impression. Both sides are marked by a number or (more rarely) a letter. This mark stands at the right-hand side at foot of first page and is called the first signature; the second signature takes the same position on the second side of the sheet, that is, on page 3. Take any book haphazard and you will find the true signature on the first and the second signature on the perfecting impression, always in the same position. In order to show at a glance whether the signature is the first or the second, the compositor adds an asterisk, thus--
We will see later that there is still another reason for the signature in the place mentioned.
When the sheets are to be folded, all the sheets of an impression are laid one on top of the other and all lying the same way. The modern quick-printing presses gather the sheets automatically, so that a sheet is rarely laid the wrong way, excepting through want of care in looking over them, or in the knocking-up in the printing office a few sheets or a whole batch are thrown out of order. According as we are dealing with folio, 4to, or 8vo _format_ we must order the gathering of the sheets and, of course, the position of the signature. The commonest _format_ is 8vo, that is to say, the size which gives eight printed leaves (or 16 pages) to the sheet. At the same time, the method of folding this _format_ is the most economical and includes the others as well.
When folding 8vo sheets the pile lies in front of the worker in such a way that all the second signatures lie uppermost at the foot of the outside right page and all the first signatures at the left on the under side of the sheet; or, in other words, all first-side leaves are turned face downward and perfecting impressions face upward.