The 2,500 words fill a page that is a couple of inches shorter than this one, but almost as wide. The faded letters in Latin are unreadable in places. Something that looks like a scraggly, russet-colored tail hangs from the bottom.

It is the document that laid the foundation for fundamental principles of English law. Angry colonists complained long before the Boston Tea Party that King George III had violated it. The men who drafted the United States Constitution and the Bill of Rights borrowed from it.

It is Magna Carta, agreed to by King John of England in 1215 and revised and reaffirmed through the 13th century. The tail dangling off the page is a royal seal.

And it is about to go on sale.

Sotheby’s, which today is expected to announce plans to auction it in New York in mid-December, estimates that the document will sell for $20 million to $30 million. It is the only copy in the United States and the only copy in private hands. Sotheby’s says the 16 others are owned by the British or Australian governments or by ecclesiastical or educational institutions in England.