The articles aims at reconsidering the emergence of the first statutory legal systems of Rome during the 12th century. The understanding of such phenomenon has been so far hindered by the total lack of ‘internal’ evidence such legal manuscripts, registers, and received letters. A new methodological approach has been used: the analysis of defensio formulas in private documents issued by Roman churches, where the first Roman statutory systems (bonus usus, consuetudines) are mentioned e contrario. The process is here reconstructed considering the history of Roman iudices, the so-called «legal Renaissance», and the formation of municipal Roman courts – the Senate –, as well as through a comparison with the more complete case of Pisa.