For the first time in history, and for only three days, the four surviving original copies of the Magna Carta will be reunited in the British Library during the document's 800th birthday, which will be celebrated in 2015. The document was first introduced by British nobles in 1215—800 years ago—which also explains why only 1215 select members of the public will be able to see it.
As you were surely told in your junior high history class, the Magna Carta is essentially the precursor of documents like the U.S. Constitution, putting as it does checks and balances on people in power. It is also, somewhat less relevantly, in the title of a middling Jay-Z album.
In 1215, baron pushed this document onto King John in hopes of protecting their own rights and property from royal power. At the time, King John was gaming the feudal system through a series of loopholes and abusing the reach of the crown—15 chapters of the Magna Carta were focused on that, the BBC explains. The next ten chapters had to do with finances and people's rights under Common Law. "It is these latter that have been seen as crucial, as they subjected the king to the law of the land for the first time in Britain's history," the BBC adds.