It is a single parchment sheet, filled with handwritten medieval Latin and dotted with a fragmentary royal seal resembling a chewed toffee. But it is one of the most important documents in the world — a 13th-century copy of the Magna Carta, the English legal document enshrining the rights of the individual against the state.

The hallowed parchment goes on view Friday at Sotheby’s and will be auctioned on December 18. Estimated to sell for between $20 million and $30 million, it is the only privately owned copy of the Magna Carta, and its owner is the one-time presidential candidate Ross Perot. The other 16 copies in existence are in public collections in Britain and Australia. As a result, this is likely to be the only time the Magna Carta will ever be on the market.

“When looking at when our rights began, it’s back to the Magna Carta,” Sotheby’s vice chairman, David Redden, said. “It makes the ideas of freedom and democracy much more tangible because you can point to the document where they started.”

In June 1215, King John assented to the Magna Carta — Latin for “Great Charter”— in Runnymede, a meadow on the banks of the River Thames outside London, in order to pacify his rebellious barons. The charter’s declaration that “No free man shall be taken or imprisoned … except by the lawful judgment of his peers or by the law of the land” set lasting precedents for the right to trial, trial by jury, and the presumption of innocence. King John died three months after he signed the Magna Carta but the charter was confirmed in revised versions in 1216, 1217, and 1225.

Copies of the Magna Carta were widely distributed throughout England but few have survived. Four of the original 1215 issue remain; two are kept in the British Library and one each in Salisbury and Lincoln cathedrals. The Magna Carta entered the British statute book in 1297, and it is this issue of the charter that Sotheby’s is selling. “This is the most important version, the one that entered English law,” Mr. Redden said.

That the English monarchy was not granting American colonies the rights affirmed in the Magna Carta became a lasting grievance to the framers of the Constitution. “The Magna Carta was ever central to the case against the king in the English Civil War which itself spawned the Whig movement that gave the founding principles to the Founding Fathers,” the English historian Andrew Roberts said.

The special relationship enjoyed between Britain and America extends to the site where the Magna Carta was signed. In Runnymede stand both a Magna Carta Memorial, in the shape of a domed temple, erected by the American Bar Association in 1957, and a British memorial to John F. Kennedy, dedicated in 1965 by Queen Elizabeth II and Jacqueline Kennedy. “Its significance was international because of the impact it had in other countries, and it has echoes in the American Bill of Rights as well as the Universal Declaration of Human Rights,” the head of medieval and earlier manuscripts at the British Library, Claire Breay, said. “That’s why it still gets cited by lawyers and quoted by politicians.”

The 1297 charter — one of only four of its kind in existence — has been on view at the National Archives in Washington since 1985, courtesy of the Perot Foundation. Mr. Perot purchased the document from Edmund Brudenell of Deene Park, Northamptonshire, Central England, a descendant of one of Britain’s most prominent medieval families. The Brudenell family is understood to have taken possession of the charter in the 16th century, possibly from a monastery ransacked during the Reformation. The Perot Foundation is selling the charter to fund medical research, education, and the families of wounded soldiers.

If the new owner follows Mr. Perot’s example, the Magna Carta will remain available to the public. “It’s a document that deserves to be exhibited,” Mr. Redden said. “This is not a trophy to take home and put in the vault.”

Until December 18, the Magna Carta will be on display at Sotheby’s headquarters on York Avenue at 72nd Street. If the hammer price reaches near the estimate, it will be considerably higher than the Sotheby’s 2000 sale of the Declaration of Independence for $7.4 million. Sotheby’s also sold a rare complete copy of the First Folio of William Shakespeare’s plays for $5.2 million in 2006. Mr. Redden said the manuscript was in fine state: “This is not fragile; it’s a really tough document. Mice have taken a bit of a nibble in a few places, but for a medieval document it’s in fantastic condition.”