What's 10' 4'' wide and four stories tall?

You might say George Schreiner is an expert in vertical living. Each morning, from his fourth-floor bedroom, he descends a flight of stairs to reach his closet. Another flight and he's to the bathroom. One more to get to the kitchen and out the front door.

''It's like a two-bedroom condo on its side,'' Mr. Schreiner says.

One Sunday recently, with yellow tape measure in hand, the young assistant professor at the Harvard Medical School established that the lean-looking structure was 10 feet, 4 inches wide - which might qualify it as the second narrowest house in North America.

Admittedly, the conclusion is not based on the most extensive data. The historic section of the city of Quebec boasts of having the narrowest house in North America, but that turns out to be a huge 15 feet, 3 inches.

A New York dwelling in Greenwich Village has done much better, with a slim profile of 91/2 feet, according to Alan Haber, public relations director of the city's Landmarks Preservation Commission.

(Global dimensions to the contest were squelched by the Guinness Book of World Records, which says the world's smallest residence belonged to Alexander Wortley, an Englishman who spent his last 20 years in a 5-by-4-by-3-foot box, with a small extention for his feet.)

Still, Schreiner's abode, on Hull Street near the historic Old North Church, deserves some sort of acclaim - if only for the legend that surrounds it.

It is called the Spite House - not, as one might suspect, because it forces its dwellers to clamber endlessly up and down stairs, but because of its origins.

As the story goes, several years after the Revolutionary War, three brothers divided an inheritance. The two eldest got adjacent houses set back from the street, while the youngest was saddled with a small plot in front of the houses.

This didn't sit well with the youngest, so he decided to build a house of his own on the site, effectively keeping horses and carriages from having access to his brothers' dwellings.

He used a hodgepodge of leftover materials. One side is brick, the other three are made of clapboard, and ''there's not a right angle in the place,'' Schreiner says. But ultimately, the youngest son's house would be the only one to survive to the present day.

The other two were torn down so long ago that even the neighborhood old-timers barely remember them, Schreiner says. And the demolition left him with a huge backyard - so huge, in fact, that it is perhaps the largest backyard in the surrounding area, he adds, although scientific data are lacking.

Schreiner finished painting the house last week. He changed it from black to robin's egg blue, with a cream color for the shutters. The color has brought a howl of protest from some neighbors; so far, those in favor barely outnumber the naysayers, Schreiner says.

But most everyone is willing to wait and see how it weathers, he adds.