The aim of this contribution is to clarify how the supreme court influenced the development of substantive private law in the Duchy of Milan. First of all, the Senato did not give any reason for its judgements. All its activity was surrounded by secret, the so called “arcana Senati”. However, through indirect sources, mainly consilia and reports of the Senato’s case law, it was possible to understand how the supreme court enforced private substantive law through its judgements, i.e. decisiones. Secondly, the Senato was a supreme court which did not consider its own precedents binding. “Legibus, non exemplis iudicandum est” repeated the court and echoed Lombard legal doctrine. However, the other judges of the Duchy, the ‘lower’ jurisdictions which were called “inferior” judges, were bound to the Senato’s judgements. In fact, Senato had strong control on these judges and received appeals and petitions against their judgements. Moreover, this “control” was imposed also through other mechanisms which are very important to understand how this supreme court influenced in practice the development of substantive law, and precisely private law. Every day the Senato’s record office received many supplications from the people of the Duchy who were having problems with a specific judge or a specific procedure. These requests were called preces and to these requests the Senato usually responded through its rescripta, which were a kind of letters addressed to the judge concerned. In general, Senato rescripta were provisions through which the high court sent varied instructions to inferior judges on how to decide single cases. An analysis of these provisions is essential to sketch a picture of Senato’s stylus iudicandi “in civilibus” for the almost three centuries it administered justice. As stated by Giulio Claro, a well-known 16th century Italian jurist, to learn what was the rule, what was the substantive law in force, one had to look at the Senato’s case law, which was the result of Senato’s different provisions, mainly decisions and rescripta. What emerges is a complex and lively picture, closer to everyday needs of the people than one could expect from a court judging “tamquam deus”.