In London, the Thames is more than a physical divide

As ambassador to the Court of St. James's from the newly born United States of America, the future president took up residence on Grosvenor Square in London's fashionable Mayfair district, an easy walk to the bespoke tailors on Savile Row and the royal residence at Buckingham Palace. The move launched an American presence on the square, north of the River Thames, that has lasted more than 200 years.

Not for much longer. Last month, the U.S. Embassy announced that, partly for security reasons, it was going to pull up stakes from one of London's smartest enclaves in exchange for a diplomatic compound to be built in the Nine Elms neighborhood just two miles away, as the crow flies.

That crow, however, would be flying across the Thames to London's south side, and therein lies the rub. As any resident will tell you, the river is more than just a physical divide between the British capital's upper and lower halves; it's a psychological and social one as well.

As I discovered even before moving here recently, which side of the Thames you live on truly puts you on the map -- not just the indispensable A-Z street guide found in every household, but the mental atlas Londoners consult to locate you on the spectrum of sass and class. Your address lands you on one side of a geographical rivalry that's full of pride and prejudice.

The north is, without doubt, classic London, home to Buckingham Palace, the houses of Parliament, Hyde Park and long-established, phenomenally expensive enclaves with names like Belgravia, Mayfair and Kensington. You can hardly walk a block without spotting a circular blue plaque marking the building where some historically significant politician, inventor or poet once lived. The city's theater district, museums, concert halls, posh restaurants and shopping meccas such as like Harrods lie in the north.

Ask a north Londoner about the other side of the Thames, and he may well sniff that it's "a bit common really." One blog describes south London as the "wasteland where plague victims were buried," adding, "Very little has changed."

Not surprisingly, southerners don't take kindly to such superior sighs. They see their northern neighbors as snobs too stuffy to appreciate the merits of south London, which may lack Old World elegance but buzzes with the energy and edge that a new wave of residents has brought in.