To most newcomers, the most befuddling geographic feature about Washington is our diagonal avenues, which seem to them to interrupt the right-angle street system that is normal in most American cities. Actually, as we locals know, the diagonals are the key to getting around fast.

But one newcomer, a man who has just moved here from a Pittsburgh suburb, says he's utterly baffled by Washington's street-numbering system. He works near Thomas Circle -- where 14th Street, M Street, Vermont Avenue and Massachusetts Avenue converge -- and he has rented an apartment in Arlington.

Driving downtown on his first day of work, having crossed Key Bridge and traversed the Whitehurst Freeway, he found himself on K Street. And what did he find?

"Well, I passed 19th Street, 18th Street, 17th Street, 17th Street, 16th Street, 15th Street, 15th Street, then 14th Street . . . . "

Hey! Wait a minute. Two 17th streets, two 15th streets?

Yes.

Blame, if you want, Pierre L'Enfant and Benjamin Banneker. In laying out this city, they interrupted the diagonal Connecticut Avenue between I and K streets with what later became known as Farragut Square; the streets a block apart that flank Farragut Square, on a north-south axis, are both called 17th Street NW. Likewise, the mapmakers interrupted the diagonal Vermont Avenue between I and K streets with what we now call McPherson Square; the streets a block apart, on a north-south axis, are both called 15th Street.

Metro Scene is waiting for this newcomer to find 13 1/2 Street NW, or Half Street SE. Or for him to discover that there are 15th and 17th streets NE and SE as well -- but, in the latter cases, just one of each. (Those streets SW are nonexistent; they'd be under the Potomac River.)