We often find writers, when referring to ancient documents, making use of the words parchment and vellum as if the terms were synonymous; but this is not strictly correct. It is true that both are prepared from skins, but the skins are different. They are similar, but not the same, nor, indeed, are they interchangeable. In point of fact, the skins of almost all the well-known domestic animals, and even of fishes, have been used for the purpose of making a material for writing upon. Specifically among the skins so prepared were the following: the ordinary lambskin, called "aignellinus"[2]; that prepared from stillborn lambs, called "virgin parchment." [2] Strictly _agnellinus_. From sheepskins was produced ordinary "parchment," and also a sort of leather called "basane" or "cordovan." Vellum was produced from calfskin; that of the stillborn calf being called "uterine vellum," and considered the finest and thinnest. It is often spoken of in connection with the exquisitely written Bibles of the thirteenth and fourteenth centuries as of the highest value. Besides these were the prepared skins of oxen, pigs, and asses; but these were chiefly used for bindings, though occasionally for leaves of account and other books liable to rough usage. Before the tenth century the vellum used for MSS. is highly polished, and very white and fine. Afterwards it becomes thick and rough, especially on the hair side. In the examination of certain MSS. the distinction of hair side and smooth side is of importance in counting the gatherings so as to determine the completeness, or otherwise, of a given volume. Towards the period of the Renaissance, however, the vellum gradually regains its better qualities. Thus it may be seen that the difference between vellum and parchment is not a mere difference of thickness; for while, in general, vellum is stouter than parchment, there is some vellum which is thinner than some parchment. Not only are they made from different kinds of skin, but the vellum used for illuminated books was, and still is, prepared with greater care than the parchment used for ordinary school or college treatises, or legal documents.