While many may praise the Chapter House, built at the same time as the cloisters and finished in 1284, for being a
most delicately beautiful, light-filled, fan ceilinged, octagonal chapter house (now that's a mouthful of adjectives), its main points of interest are the historical relics that it contains.
The most important of these relics is one of four surviving copies of the Magna Carta. It sits in a case on the opposite side of the huge room designed after the Chapter House at Westminster Abbey. One word of warning: while pictures are allowed in all other parts of the cathedral, neither these nor videos are allowed here, even without flash.
The Magna Carta was a charter signed by King John in 1215 and was a predecessor to such documents as the American Bill of Rights. It came about because many English barons felt that the king was abusing his power and captured the city of London in June of that year; these barons then forced the king to sign the "Articles of the Barons" in a meeting at the meadow of Runnymeade. No one knows quite how many copies of this document were made to send to the various barons and other high-ranking officials, but as I've stated above, it is known that only four still exist, and none that bear the royal seal.
The charter was supposed to limit his rights and make him respect the law. However, throughout the Middle Ages, it did not quite serve this function, and by the 1800s, the majority of the charter was no longer part of British law. In fact, King John had the charter revoked as soon as the angry barons left, and the Pope even renounced it, saying it was "a shameful and demeaning agreement, forced upon the king by violence and fear." Luckily, the Bill of Rights has had a slightly better fate!
The guide standing next to the case holding the charter, written on vellum, was very knowledgeable and answered everyone's questions about the charter and its origins. More people than I ran into in the entire cathedral seemed to be crowded around the case, politely jostling for a look at the charter. It did take quite a close look to actually read the charter, since it was written in tiny, perfect hand.
Many people stopped to look at the many other relics in the Chapter House, but I didn't dilly-dally too much longer after seeing the Magna Carta, being a bit tired from a long day of touring. You could definitely spend more time than I did in there though, especially if you are a history buff.