Duchsustus (דוכסוסטוס, from Greek δυσχιστός dyschistos) is the name of a type of parchment used for religious writings in Judaism. It is originally a Greek word and one of three Talmudic names for animal skin. The other two are kelaf and gevil. The meanings of these terms, however, are the subject of controversy in Jewish law. According to the Talmud, a sefer Torah should, ideally, be written on gvil, but may also be on klaf, Tefillin must be written on Klaf, and Mezuzah should be written on duchsustus, kelaf, or gevil. This instruction is dated to Moses at Mount Sinai. Duchsustus is the animal's dermis, klaf is the epidermis, and gevil is both layers tanned unseparated.