People throughout the world have heard of Magna Carta. It is often held to be the corner-stone of liberty and the chief defence against arbitrary and unjust rule in England. It contains few sweeping statements of principle, but is a series of concessions wrung from King John by his rebellious barons in 1215. Magna Carta was promptly overturned by the Pope, to whom John had given the overlordship of England. Four copies of the original version survive. Two are held in the British Library; the others are in Lincoln and Salisbury.

http://www.bl.uk/onlinegallery/highlights/tour/magnacarta.html

here is a law which is above the King and which even he must not break. This reaffirmation of a supreme law and its expression in a general charter is the great work of Magna Carta; and this alone justifies the respect in which men have held it.

--Winston Churchill, 1956

King John of England agreed, in 1215, to the demands of his barons and authorized that handwritten copies of Magna Carta be prepared on parchment, affixed with his seal, and publicly read throughout the realm. Thus he bound not only himself but his "heirs, for ever" to grant "to all freemen of our kingdom" the rights and liberties the great charter described. With Magna Carta, King John placed himself and England's future sovereigns and magistrates within the rule of law.