The method of textual criticism which has been generally practised by editors of classical Greek and Latin texts involves two main processes, recension and emendation. Recension is the selection, after examination of all available material, of the most trustworthy evidence on which to base a text. Emendation is the attempt to eliminate the errors which are found even in the best manuscripts.
The application of critical methods in the editing of classical texts was developed principally by three German scholars, Friedrich Wolf (1759-1824), one of the founders of classical philology, Immanuel Bekker (1785-1871), and Karl Lachmann (1793-1851). Bekker devoted his long life to the preparation of critical editions of Greek texts. Bekker collated some 400 manuscripts, grouped existing manuscripts of an author into families where one was derived from another, and published sixty volumes of improved editions of Greek authors. Lachmann went further than Bekker, showing how, by comparison of manuscripts, it is possible to draw inferences as to their lost ancestors or archetypes, their condition, and even their pagination.
The basic principle which underlies the process of constructing a stemma, or family tree, of manuscripts is that, apart from accident, identity of reading implies identity of origin. Often, however, difficulties hinder the construction of a stemma of manuscripts. A disturbing element enters when mixture has occurred, that is, when a copyist has had two or more manuscripts before him and has followed sometimes one, sometimes the other; or, as sometimes happened, when a scribe copied a manuscript from one exemplar and corrected it against another. To the extent that manuscripts have a “mixed” ancestry, the genealogical relations among them become progressively more complex and obscure to the investigator.
Metzger, The Text of the New Testament, pp. 156-159.