Any flat surface for inscriptions, especially those on which cuneiform inscriptions were written. Tablets were normally of clay but were also made of stone or metal. The shape and size varied according to the nature of the inscription and the period when the tablet was inscribed. An impressed tablet is one bearing notations impressed with tokens or the blunt end of a stylus. These tablets were referred to in literature as "numerical tablets" as they noted units of goods. An incised tablet bears notations traced with the sharp end of a stylus. A pictographic tablet is one bearing notations traced with the sharp end of a stylus; these two types of tablets had signs in the shape of the things they represented. The earliest known books are the clay tablets of Mesopotamia (and the papyrus rolls of Egypt)dating from the early 3rd millennium BC.