what is it?
A Constitutional document
Magna Carta
 "the greatest constitutional document of all times – the foundation of the freedom of the individual against the arbitrary authority of the despot". 
how did the magna carta advance the cause of liberty Magna Carta still forms an important symbol of liberty today, often cited by politicians and campaigners, and is held in great respect by the British and American legal communities, Lord Denning describing it as "the greatest constitutional document of all times – the foundation of the freedom of the individual against the arbitrary authority of the despot".
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Magna Carta [SEP] Introduction
In the 21st century, four exemplifications of the original 1215 charter remain in existence, two at the British Library, one at Lincoln Castle and one at Salisbury Cathedral. There are also a handful of the subsequent charters in public and private ownership, including copies of the 1297 charter in both the United States and Australia. The original charters were written on parchment sheets using quill pens, in heavily abbreviated medieval Latin, which was the convention for legal documents at that time. Each was sealed with the royal great seal (made of beeswax and resin sealing wax): very few of the seals have survived.