Jersey and the other Channel Islands were part of the Duchy of Normandy from 933 to 1204 when King John of England lost continental Normandy to Philippe Augustus of France. The ancient custom of Normandy is accordingly the basic law of the Bailiwicks.
The custom crystalised c 1060 but the oldest surviving text, Le Très Ancien Coutumier was written c 1200.
This was followed by the Summa de Legibus Normannie, better known in its French version as Le Grand Coutumier, a classic work still quoted as authority in Jersey.
The most famous of the commentators on Le Grand Coutumier were Rouillé C1534 and Terrien c1574.
The ancienne coutume was reformed in 1583 (La Coutume Reformée) and adopted by the Parliament of Rouen by Royal Edict as the law of the province. The Norman custom was steadily modified by the introduction of Roman Law, which revolutionised procedure and supplied elements of contract law reviously lacking. The custom was largely abolished and replaced by a new Code when the Ancien Régime came to an end in 1789.
In Jersey, in contrast, the custom remained more conservative but has continued into the 21st century, albeit much modified recently by statute and in earlier centuries by successive Orders in Council. English common law has invaded large areas of contract and tort. In the case of land law, however, many of the governing principles are still based on ancient custom.