Vorlage
A Vorlage (; from the German for "prototype" or "template") is a prior version or manifestation of a text under consideration. It may refer to such a version of a text itself, a particular manuscript of the text, or a more complex manifestation of the text (e.g., a group of copies, or a group of excerpts). Thus, the original-language version of a text which a translator then works into a translation is called the "Vorlage" of that translation. For example, the Luther Bible is a translation of the Textus Receptus, so the Textus Receptus is the "Vorlage" of the  Luther Bible. Sometimes the "Vorlage" of a translation may be lost to history. In some of these cases the "Vorlage" may be reconstructed from the translation. Such a reconstructed "Vorlage" may be called a "retroversion", and it invariably is made with some amount of uncertainty. Nevertheless, the "Vorlage" may still be reconstructed in some parts at such a level of confidence that the translation and its retroversion can be used as a witness for the purposes of textual criticism. This reconstructed "Vorlage" may stand on its own as the sole witness of the original-language text, or it may be  compared and used along with other witnesses. Thus, for example, scholars use the reconstructed "Vorlage" of the Greek Septuagint translation of the Hebrew Bible at parts to correct the Hebrew Masoretic version when trying to determine oldest version of the Hebrew Bible that they can infer. Or, as another example, the Coptic fragments of Plato's Republic included among the Nag Hammadi library are used to help attest to the original Greek text which Plato himself wrote. For the bulk of the Gospel of Thomas, the "Vorlage" exists only as a retroversion of the Coptic translation, as no other witness  to the original Greek text for most of the sayings recorded therein is known.