Unwritten law, or ius non scriptum, was a prominent definition of custom in the later medieval period developed especially in learned circles. Though ‘written custom’ should be a contradiction, this was unevenly perceived. Unlike the authors of the Glanvill and Bracton lawbooks in England, the coutumier authors did not seek to understand or legitimate their writing of custom within this learned framework. Those coutumier authors who addressed the question of writenness viewed the idea of ‘written custom’ as unproblematic. Instead, they saw writtenness and its lack through a practical lens, as an aspect of custom’s potential to be retained in memory. This does not mean they viewed written custom as ‘fixed’. Writing in a time of fundamental legal change, they understood that even the custom expressed in written text was mutable. While writing custom could help to keep it memory, it was potentially changeable in future versions of the text.