IN rege qui recte regit necessaria sunt duo hæc, arma videlicet et leges, quibus utrumque tempus bellorum et pacis recte possit gubernari. Utrumque enim istorum alterius indiget auxilio, quo tam res militaris possit esse in tuto, quam ipsæ leges armorum podio sint servatæ. Si autem arma defecerint contra hostes rebelles et indomitos, sic erit regnum indefensum: si autem leges, sic exterminabitur iustitia, nec erit qui iustum faciat iudicium. Cum autem fere in omnibus regionibus utatur legibus et iure scripto, sola Anglia usa est in suis finibus iure non scripto et consuetudine. In ea quidem ex non scripto ius venit quod usus comprobavit. Sed non erit absurdum leges Anglicanas licet non scriptas leges appellare, cum legis vigorem habeat quidquid de consilio et consensu magnatum et rei publicæ communi sponsione, auctoritate regis sive principis præcedente, iuste fuerit definitum et approbatum. Sunt etiam in Anglia consuetudines plures et diversæ secundum diversitatem locorum. Habent enim Anglici plura ex consuetudine quæ non habent ex lege, sicut in diversis comitatibus, civitatibus, burgis et villis, ubi semper inquirendum erit quæ sit illius loci consuetudo et qualiter utantur consuetudine qui consuetudines allegant.

 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 defense; if laws fail, justice will be extirpated; nor will there be any man to render just judgment. 

 Though in almost all lands use is made of the leges and the ius 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.1 c.5

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