The U.S.-Canada border is known as the longest undefended frontier on Earth. The two countries have long been regarded as good neighbors who share a relationship unique in the world.

That such notions ignore countless economic, political, environmental, territorial and even military squabbles over the years is beside the point. From coast to coast, monuments to American-Canadian amity adorn the boundary: the Peace Arch, the International Peace Garden, the Peace Bridge.

But no public institution along the 5,525-mile border more embodies this friendly coexistence, or has more vested in keeping the peace intact, than the Haskell Free Library and Opera House, which literally straddles the line, with part of the building standing on American soil, part on Canadian. As such, the Haskell has at least two of almost everything.

Two addresses (7 Caswell Ave., Derby Line, Vt. 05830; 1 Church St., Rock Island, Quebec J0B 1K0).

Two telephone numbers (802-873-3022; 819-876-2020).

Two languages (English; French).

Two doorways (front entry in the United States; back fire exit in Canada).

And two disparate functions: Upstairs is a charming opera house, with its stage in Canada and most of its seats in the United States. Downstairs is a robust community library, with its lobby in Vermont, its book stacks in Quebec and its reading room in both.

That the yellow-brick-and-gray-granite building is astride the border is no accident. Nearly 100 years ago, Martha Stewart Haskell--with the help of her son, Horace--had it built there in honor of her late husband, Carlos, a prominent local merchant. He had been American, she was Canadian. She had friends on both sides of the border, didn't want to insult any of them but did want to erect a cultural center to serve both communities. Her plan was that revenue from shows at the opera house would support the library. She hired an eminent Quebec architect and his Boston partner to design the building, enlisted a Quebec company to construct it largely of materials indigenous to the area, and spent at least $50,000--a huge sum then in a small town--before the place was completed in 1904.

"It was a gesture of friendship, a good deal less symbolic than most such gestures," Canadian author Marian Botsford Fraser has written. "Also, it was conceived by a person, not a bureaucratic group, and therefore is idiosyncratic to a degree unimaginable by a committee."

A committee, for instance, might have balked at the smack-on-the-boundary-line construction, even though it was not an altogether uncommon practice among private businesses and homeowners in a day when the border was not as precisely defined, clearly marked or strictly enforced as it is today. And a committee probably wouldn't have gone for the eccentric opera house/library combo either. Which would have been a shame, because the Haskell may be the only building of its kind in the world.

To enter the 400-seat opera house is to visit an era gone by. Its turn-of-the-century detail is breathtaking: the proscenium arch, the plaster cherubs that ring the balcony, the high pink-and-white ceiling made of pressed tin, the original brass chandelier, the hand-painted linen curtain that rolls up manually by cord and pulley, the stunning stage scenery. All in virtually mint condition--partly because the opera house itself is closed in winter and left unheated, which has aided preservation. The curtain and three sets of scenery, according to local historian Matthew Farfan, are believed to be the only surviving works of Erwin LaMoss, a renowned Boston artist.

Martha Haskell's idea to fund the library with profits from stage productions never panned out, but the opera house is open from late April to early October. This season included approximately 40 shows by performers ranging from the Montreal West Operatic Society, the I Musici chamber orchestra of Montreal and the Vermont Symphony Orchestra to local musicians, theater companies and student groups.

The first thing you notice about the library itself is that it's a working civic resource, not a museum. Its shelves tend to be cluttered, bulging with a collection of about 20,000 books (20 percent of them in French). The checkout desk is busy. All told, it serves some 2,500 regular patrons (roughly half of them Vermonters, 30 percent English Quebecers and 20 percent French Quebecois), five days a week, year-round. Soon you gravitate toward the pleasant, airy reading room with its stained-glass windows inscribed with the names of Haskell family members, its ornate fireplace and its native birch woodwork.

At some point you focus on the thin black stripe that cuts diagonally across the hardwood floor and demarcates the international boundary, this portion of which was established in 1842 by the Webster-Ashburton Treaty between the United States and Britain (of which Canada was then a province). While a handful of houses and one private factory in Derby Line/Rock Island also straddle the line, the Haskell is the only public building in town--and the only structure of its kind along the entire U.S.-Canada border--to do so.

Officially, Ottawa and Washington have declared the building a "no man's land." Patrons from surrounding communities are not required to report to customs before or after using it. The Haskell pays taxes to neither country, a stipulation the family required in donating it to the community. It is technically a Vermont nonprofit organization, but an international board of trustees (four Americans, three Canadians) oversees its operation. Today, the Haskell could not be built where it is, because the International Boundary Commission has effectively banned on-the-border construction since 1925.

Still, the Haskell's binational positioning has engendered some legal peculiarities and logistical nightmares.

The reading room has hosted at least three court proceedings in which either a witness or a defendant was unwilling or unauthorized to leave or enter Canada or the United States. All three hearings involved smuggling of some sort--drugs, illegal aliens or bombs--according to Farfan.

Similarly, in the early '70s, when John Lennon, Paul McCartney and George Harrison were "undesirable aliens" in one country or another, legend has it that the Beatles considered a business reunion at the Haskell, wherein Lennon would have stayed on the U.S. side, McCartney and Harrison would have stayed in Brit-friendly Canada, and Ringo Starr, who had no outstanding legal offenses, apparently would have been free to cross the line as he pleased. When word of the reunion leaked to the media, according to local lore, the idea was scrapped for fear of a mob scene.

There have been several weddings, most for legal reasons involving mates of differing citizenship, but there was one ceremony a while back in which both halves of the happy couple were American and the bride had to walk up the opera house aisle, not down, so the vows could be spoken on U.S. territory.

The most recent logistical nightmare began in 1993, when fire inspectors closed the Haskell for safety violations. The library was quickly brought into compliance and reopened, but it took four years and $600,000 in renovations before the opera house was able to satisfy all of the national, provincial, state, regional and municipal agencies involved.

Its spot on the border is not all bad from a practical perspective, though. Because the Haskell has an oil tank on each side of the line in the basement, it buys heating fuel from the cheapest supplier based on exchange rates and market conditions in both countries. Because it has access to utilities in Vermont and Quebec, it does the same with electricity.

For some townsfolk, there's a less tangible advantage, too. "I can be home and at home in the same place," says Miriam Klein Hansen, a Haskell librarian who is a Montreal native but has lived in Vermont for 20 years. "That's the biggest appeal about the building for me."

The Haskell Free Library and Opera House is about 100 miles southeast of Montreal, about 220 miles northwest of Boston and about 600 miles from Washington. But it's not far off the superhighway, just a half-mile from the border point where Interstate 91 becomes Quebec Autoroute 55. Tours are offered Tuesday through Saturday from May through October (subject to availability; $2 donation suggested).