At the beginning a tapestry of laws of localities, from mid (15th there are steps being made towards a "modern" national legal system.
A north-south divide (La Rochelle-Geneva) can be traced in early French law. In the south (sometimes called "le pays de droit ecrit") the influence of Roman law remained strong; to the north the influence of the Germanic system was stronger.
In 1454, ordinance of Charles VII decreed that a written record should be made of all existing oral local custom. (Coutumiers term for collections of Coutumes locales.)
Under Francois I (and Chancellier Poye) the Ordonnance de Villers-Cotterêts (1539) introduced reforms across a wide range of legal matters, including criminal procedure.
Gallica - the online catalogue for the digitised collection of the Bibliotheque Nationale de France - is an excellent place to find early editions of the various coutumiers available on the free web.
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Customary law - France
Law France History
Law France Sources
Feudal Law -France
Land tenure -- Law and legislation -- France -- Early works to 1800
Feudalism - France