The customary law of Guernsey (Coutume) was and still is the law of Normandy as adapted and modified in Guernsey over the centuries.
In the 1570s there was local disquiet about a lack of certainty as to prevailing law. A Royal Commission appointed in 1579 considered the extent to which the customs and laws of Normandy applied in the Island. The Royal Court was directed by the Commission to follow the law as stated in the 13th century Grand Coutumier de Normandie to the extent that it covered matters not dealt with in documents known as the Extente (of Royal Revenues) in 1331 and the Precepte d'Assize of 1441.
The Royal Court in response to the Commission's views produced a further document referring to Guillaume Terrien's 1574 "Commentaires du Droit Civil tant public que privé, observé au pays et Duche de Normandie" being the most up to date commentary on the Law of Normandy at that time. The court had analysed Terrien's work and it set out the differences between Terrien's statement of the Law of Normandy and the Law as practised in Guernsey. In 1583 the Royal Court submitted this work to the Privy Council and the Privy Council that year
"ratified and approved ... the Laws and Customs therein contained"
with certain caveats as to future change. The document to this day is known as the Approbation des Loix.
Since that time the Coutume as applicable in Guernsey has evolved and been the subject of Court interpretation.
Nowadays elements of the Coutume survive particularly in the fields of Land Law and to a lesser extent Inheritance Law but most of the law of Guernsey is to be found in legislation which has been enacted to meet modern needs across the whole spectrum of life and commerce, some of it covering areas of law unknown in the Duchy of Normandy.
Legislation is contained in Royal Charters, Orders in Council, Acts of Parliament extending to Guernsey, Ordinances and Statutory Instruments and also Regulations of the European Community applicable in the restricted area covered by Protocol No. 3 to the United Kingdom's 1972 Treaty of Accession to the European Economic Community. A considerable number of international treaties and conventions have also been extended to Guernsey and Guernsey has also entered into a number of bilateral agreements.