In many ways, it is a document that made history. In June 1215 in a muddy field by the Thames just outside Windsor, King John affixed his seal to an agreement known by its Latin name, Magna Carta (literally, 'big charter').
At the time, it was essentially a peace treaty; John was widely discredited by 1215, having lost Normandy eleven years earlier and subsequently levied excessive taxes to fund his unsuccessful attempts to get it back. His barons were in open revolt, and faced with the prospect of a French invasion John had little option but to agree to their demands. The charter that he sealed (not signed) regulated the administration of justice and established the principle of due legal process.
John, of course, was not one of England's more reliable kings and he went back on his word as soon as he could. What saved Magna Carta was his death a year later; after that, a revised version was issued to win support for the new King, Henry III, who was just nine years old. Subsequent reissues in 1217 and 1225 ensured that Magna Carta was imprinted on the consciousness of the nation; among its key points were the right to a fair trial - which is still on the statute books to this day:
"No free man shall be seized or imprisoned, or stripped of his rights or possessions, or outlawed or exiled, or deprived of his standing in any other way, nor will we proceed with force against him, or send others to do so, except by the lawful judgement of his equals or by the law of the land."
This was one of the first steps taken in England towards the establishment of parliamentary democracy. Over time, it was used by those wishing to restrain royal power (those who drew up the Petition of Right and the Grand Remonstrance in the run-up to the Civil War were inspired by it, and it was cited at Charles I's trial), and it greatly influenced the American colonists' Declaration of Independence in 1776. In the twentieth century it inspired the Universal Declaration of Human Rights. Its influence has been truly widespread, to the extent that it is said to compete with the English language as this country's greatest export.
To celebrate the 800th anniversary of the Magna Carta, there is an exhibition at the British Library which includes the four original surviving copies and looks it its long legacy. It's definitely worth going to see, although it closes at the end of the month. Go now, while you still have the chance.