This article retraces the history of the relationship between two normative techniques whose competition became clear from the end of the 18th century: "law" and "measure". Based on a comparative analysis of French and German sources of the last two centuries, the author focuses especially on the concept of "mesure de police" or "Polizei Massnahme" in order to discover the very character of an institution, the police, which does not recognize the classical distinction of continental legal systems between law and fact. Despite of the guarantees offered by a State based on the rule of law, police claims "free hands", namely the ignorance of the law. Thus its power is grounded on a relative continuity between the rule and the excess, the taming of the situation being the real criterion the police force is led by. Recent events like the misuses of police during the G8 in Genoa in 2001 confirm the long lasting history of an institution which can hardly find a compromise with the idea of limit.