In this paper I compare two field studies of customary law in action. Minangkabau in Western Sumatra is home to the largest population with matrilineal property transmission rights in the world. I show how customary law, the so-called »adat«, has been an essential part of the identity of this population, next to Islamic law, since the 17th century. Adat was also shaped by the efforts of the Dutch colonisers to write it down. My second case is set in the French medieval town of Beauvais at the turn of the 12th century, when the town was thriving. I focus on one judicial conflict surrounding a water-mill. The document pertaining to this case is the oldest to provide information about customs in Beauvais. This document illuminates the evolving legal consciousness of competing groups in the city and the process by which a medieval judge wrote custom down.