The form of books changed from a scroll to a codex in the second half of the 4th century. A codex was produced by piling up and stitching together several small booklets. Four sheets of animal skin or paper, laid one on top of another, are folded along the center line to produce an 8-leaf, 16-page booklet. This booklet is called a "quaternion," or a "quaer" or "quair" for short. This was the basic unit for bookbinding in the medieval period and is called a "quire" in English. Quires did not always have 8 leaves.
When book printing first began, large sheets of paper were folded several times to produce quires. To give instructions to the bookbinders on the order of how each quire should be laid on top of another, symbols such as A1 and A2 were printed on the lower right corner of the leaves that comprised the first half of the quires. These symbols are called the "signature (Signatur)."
For details, see Chapter 3.