Vellum is prepared animal skin or membrane, typically used as writing material. It is distinguished from parchment when it is made from calfskin, rather than that made from other animals, or having a higher quality when it is not. Vellum is prepared for writing and printing on and used for single pages, scrolls, codices, or books. Modern scholars and experts use the general and potentially-confusing term "membrane" more often. It may be very hard to determine the animal species involved (let alone its age) without going to a laboratory depending on the way of making the paper. Thus the term avoids the distinction between vellum and parchment. Vellum is generally smooth and durable, but there are great variations in its texture which are affected by the way it is made and the quality of the skin. The making involves the cleaning, bleaching, stretching on a frame (a "herse"), and scraping of the skin with a crescent-shaped knife (a "lunarium" or "lunellum"). To create tension, the process goes back and forth between scraping, wetting and drying. Scratching the surface with pumice, and treating with lime or chalk to make it suitable for writing or printing ink can create a final look. Modern "paper vellum" is made of synthetic plant material, and gets its name from its similar usage and high quality. It is used for a variety of purposes including tracing, technical drawings, plans and blueprints. The word "vellum" is borrowed from Old French vélin 'calfskin', derived in turn from the Latin word vitulinum 'made from calf'. However, in Europe, from Roman times, the word was used for the best quality of prepared skin, regardless of the animal from which the hide was obtained. Calf, sheep, and goat were all commonly used, and other animals, including pig, deer, donkey, horse, or camel were used on occasion. The best quality, "uterine vellum", was said to be made from the skins of stillborn or unborn animals, although the term was also applied to fine quality skins made from young animals.