Comparative analysis of different textual variants or copies (i.e. witnesses) is a prerequisite to establishing the attested renditions of a given source. It is the first step in reconstructing the history of a text and its dissemination. Comparing extant witnesses helps to draw a ‘family tree’ or stemma codicum, required to express the postulated interrelationships between the different manuscripts. Textual criticism is an essential component of textual editing.