The Lincoln Cathedral Magna Carta, a copy of the famous 1215 document that enshrined the rule of law in England, is on display at the Library of Congress in Washington DC.
Princess Anne opened the exhibition, stating in a speech: "Nearly 800 years ago, Magna Carta gave us our first concept of a society governed by the rule of law -- a major step.
"I think anniversaries are an opportunity to look forward to what will be happening in maybe 100, 200 years.
"Will we still be celebrating Magna Carta as it reaches those milestones? It is imperative for us to instill these values, this understanding in the next generation."
The piece is one of only four surviving original copies, one of which was sold for $21.3m at Sotheby's New York in 2007.
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