There's a type of urban housing that is more versatile than rowhouses, more human-scaled than apartment buildings and far denser than single-family homes. It's called the plex-but unless you've lived in a select few cities, you've probably never heard of it.

What exactly is a plex? Basically, it's a two- or three-storey building with one or two apartments per floor. Each apartment extends from the front of the building all the way to the back and, most importantly, each one has a more or less direct connection to the street-no lobby and no hallways, in other words. Plexes originated in Europe; in North America, they can be found most commonly in older American cities like Boston, Chicago, parts of New York and some smaller Midwest cities. Only in Montreal, however are they the predominant form of housing: two out of three Montrealers live in an apartment in a building with fewer than five storeys. Walk down a street on opposite ends of town-say, Verdun in the southwest and Rosemont in the northeast-and you'll encounter row upon row of plexes.

Classic triplexes in Montreal's Plateau Mont-Royal neighbourhood.

Older plexes from the turn of the last century, in the Centre-Sud area.

From the outside, the plexes in Boston (known as "triple-deckers") and Chicago ("three-flats") look like giant single-family houses or ordinary apartment buildings. Montreal's plexes, however, are strikingly unique. Outdoor staircases spiral their way up to a second-floor landing, from which upper-floor apartments are accessible. Balconies abound: most plex apartments have one at the front and one at the back.

According to David Hanna, an urban geographer at the Université du Québec à Montréal, these idiosyncrasies are the products of two distinct cultural influences. The outdoor staircases come from Scotland and western France - they are common in Edinburgh, for instance-and are evident in some of Montreal's oldest plexes. The balconies, however, didn't appear until the turn of the twentieth century, when a great wave of migrants from rural Quebec washed into Montreal. The transplanted country-dwellers, used to traditional French-Canadian homes with large verandas, demanded a way to escape the confines of their apartment without leaving home.

The bulk of Montreal's plexes date back to the early twentieth century, but the roots of their preponderance can be traced back to the nineteenth century, when, fearful of squalid Glasgow- and New York-style tenements, city officials banned the construction of apartment buildings. The outcome was a lesson in cultural differences: while the city's middle-class francophone immigrants preferred the plex, anglophones stuck with English-inspired rowhouses. But plexes offer a multitude of advantages over rowhouses and apartment buildings. "Apartments tend to be more cookie-cutter," notes Hanna. "It's a very caged existence." Narrow rowhouses, meanwhile, often offer the same amount of living space as a plex flat on two or three floors instead of one.

Plexes are inherently versatile, with an almost infinite number of possible apartment combinations. A classic sixplex, for instance, has two apartments per floor; a fiveplex has one apartment on the ground floor and two on each level above; and a triplex has one apartment per floor. More exclusive plexes, like those facing Montreal's Parc Lafontaine, have one flat on the bottom and a large, two-storey apartment on top. The design of each apartment can vary, too. Most triplex flats have six or seven rooms, including three bedrooms, a kitchen, a living room and an office, but other plex apartments can be much smaller or larger. L-shaped plexes extend all the way to the back of the property but their shape still allows light to penetrate into the deepest heart of the apartment.

De Lorimier Street is lined by triplexes.

A newly built "condoplex."

A triplex on Esplanade Street in Mile End neighbourhood.

Plexes have a custom-built appeal for both homeowners and renters. Many plexes cost about the same as a single-family house, so owners occupy one of the apartments and rent out the others to subsidize their mortgage. (In Chicago, three-flats are popular because owners can claim both homeowner and landlord tax deductions on the same piece of property.) This unique arrangement has provided a boost to generations of Montreal immigrants. In plex-filled Mile End, Jewish immigrants in the early twentieth century bought plexes from departing Anglo-Protestants; fifty years later, Greek, Portuguese and Italian immigrants became tenants and landlords in those same plexes, as the Jewish community shifted west. This ebb and flow of new arrivals creates what Hanna calls the "layer-cake effect": the resident landlord is a more established immigrant while the tenants are more recent arrivals, usually from different ethnic groups. Neighbourhoods thus become more thoroughly diverse, and there isn't quite as much segregation along ethnic or cultural lines. "It reduces cultural tensions," explains Hanna.

One of the greatest things about living in plexdom is the street life. Plexes- especially Montreal's plexes - are extremely conducive to what urban-planning visionary Jane Jacobs describes as "eyes on the street." The multiplicity of entrances, staircases and balconies maximizes the potential for interaction in the public realm, making plex-lined streets lively and interactive. There's always somebody coming and going and neighbours often stop for idle chit-chat. On warm summer evenings or sunny spring days, balconies fill up with people reading, relaxing or just watching passersby. "The rhythm of the street comes from the diversity of its people," says Dinu Bumbaru, the director of Héritage Montréal. The plex's orientation towards the street makes that diversity all the more apparent.

Consider balconies: plex-dwellers love them so much that a City of Montreal study two years ago found that the average Montrealer spends an average of eight hours per week on his or her balcony (during the warm months, of course). A passage at the end of Michel Tremblay's The Fat Lady Next Door is Pregnant, a novel set amidst the plexes of the Plateau Mont-Royal, finds the protagonist drinking spruce beer on her balcony one warm April evening:

When ... the fat woman was able to contemplate the street as much as she liked, steep herself in the images and sensations of the early spring night, scrutinize the smallest shadowy corners, recognizing, in spite of the darkness, the faces of all the neighbours watching her from their balconies, breathe deep the promises of May, and what still remained of April, time was suspended and nothing moved.

The "eyes on the street" phenomenon in action.

A balcony used to pass a joint ...

Chatting with neighbours ...

... and hanging out with friends.

Ensuring that the plex maintains the elements that make it so successful takes care and vigilance. The plex's high degree of craftsmanship, and unique architectural details such as wrought-iron balconies and stained-glass windows, make its preservation a vital concern. "A triplex is not a monument in itself," remarks Bumbaru. "It's really an urban landscape." And that's the gist of it: plexes make for great streets and great neighbourhoods. What has already been built must be well-maintained, but even more important is that the plex continue to exist as a viable form of housing. Luckily, developers in Montreal haven't forgotten about it: much of the city's new urban infill consists of "condoplexes" which contain many of the best elements of the traditional plex, including balconies, outdoor staircases and a direct link between apartment and street. One can only hope that these new plexes work as well as the old.

Neighbours chat below, while another watches the street above.

When not wandering our streets, Christopher DeWolf is the editor of Urbanphoto.net. The Urban Eye appears every second Wednesday.