It has long been recognized that the XII Tables was not a complete statement of the law, and that some topics of great legal importance were either not set out or were very partially treated. The general opinion has been, however, that the fragmentary state of our knowledge of the XII Tables' provisions makes it impossible to be precise as to the the topics not dealt with or treated only in part. Recently, though, I have tried to show that we have some information on the great majority of the clauses in the XII Tables: hence on this view, where we have no indication that a topic was dealt with in the code the strong presumption must be that no relevant provision ever existed. I have also argued that reasons can be found for the inclusion of many topics in the XII Tables, reasons which largely relate to the legal innovation or uncertainty in the law. When basic matters are omitted from the code this is largely because the law was well settled.
Watson, Alan, "The Origins of Usus" (1976). Scholarly Works. Paper 392.