Codices that compile different textual units in the same physical object were common in the European Middle Ages. Many reasons guided scribes when collecting a variety of works and their analysis can offer scholars insight into scribal practices and the circulation of medieval works. In order to study this shared transmission of medieval texts, methods from network analysis offer the possibility of researching the phenomenon from a general perspective and discover fundamental trends. This paper deals with the methodological and practical foundations for such an approach. Modern digital databases of medieval manuscripts provide a big amount of relevant data for this type of research. In this presentation, I consider three online databases, each dealing with textual witnesses in vernacular languages: Handschriftencensus (German) Jonas (French and Occitan) and Philobiblon (Iberian languages). Each of these has different criteria on data collection and data modelling. For this reason, their analysis and comparison offers insight not only on medieval manuscripts and texts, but also on the consequences of different approaches when cataloguing and describing medieval manuscripts.