David Letterman's TV audience would have been seriously unimpressed with David Cameron's failure to know everything about Magna Carta - because they study it in school more than the British, a Tory MP has told me.
Conservative Eleanor Laing, who is about as enthusiastic as an MP can be about Magna Carta, says that Americans are more aware of it because it's taught "as a fundamental principle of freedom in American schools", she explains.
"So an American TV audience would all jump up and laugh and say we know all about Magna Carta... the reason that it matters so much to them is that their constitution, the very creation of the United States of America, was based on Magna Carta."
This great document wasn't supposed to be in the news until June 15th 2015, when Britain will mark the 800th anniversary of this seminal moment in our history. Thanks to David Letterman, King John's deal with those pesky Angevin barons is back in the headlines a little earlier than scheduled.
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"It was at this point when King John agreed with his barons by signing the Magna Carta the constitutional significance that no one is above the law," Laing tells me.
"The rule of law prevails: the king couldn't just do what he wanted. That what they were swearing allegiance to was the charter, which sets out the principles of freedom."
Ask your average voter in the street what they might think about Magna Carta and - let's face it - they probably wouldn't know much about it. As Laing explains, the average person getting on with their daily lives takes it for granted that their constitutional rights are protected. It's only when there's a threat to them that most people will pay attention to them. With the British constitution settled and not a constant subject for debate - as it is in the US - it's no surprise they care more than we do.
Laing, as chair of the all-party parliamentary group on Magna Carta, is working closely with the American Bar Assocation to celebrate the 800th anniversary. A ceremony will take place on the island of Runnymede where the document was famously signed.
She is not especially impressed with the prime minister's failure to work out what it means in latin. "It's terribly easy," Laing explains. Magna means great or big; carta means charter, or "big piece of paper". Nevertheless, her criticism of her party leader is strictly limited.
"It's a bit difficult to deal with a question that's sprung upon you like that," Laing explains.
"I don't care whether the prime minister has the latin translation at the front of his mind. All I do care about is that he defends the principles of liberty and the rule of law. Which of course he does."