To rule well a king requires two things, arms and laws, that by them both times of war and of peace may rightly be ordered. For each stands in need of the other, that the achievement of arms be conserved [by the laws], the laws themselves preserved by the support of arms. If arms fail against hostile and unsubdued enemies, then will the realm be without defence; if laws fail, justice will be extirpated; nor will there be any man to render just judgment.

[England alone uses within her boundaries unwritten law and custom].

Though in almost all lands use is made of the leges and the jus scriptum, England alone uses unwritten law and custom. There law derives from nothing written [but] from what usage has approved.   Nevertheless, it will not be absurd to call English laws leges, though they are unwritten, since whatever has been rightly decided and approved with the counsel and consent of the magnates and the general agreement of the res publica, the authority of the king or prince having first been added thereto,  has the force of law.  England has as well many local customs, varying from place to place, for the English have many things by custom which they do not have by law, as in the various counties, cities, boroughs and vills, where it will always be necessary to learn what the custom of the place is and how those who allege it use it.

Gratian, Decretum De legibus: D.4 dictum after c.3