T he external staircases spiraling up the facades of duplexes in Montréal, Québec, Canada, provoke reactions as varied as the structures themselves.

The distinctive architectural feature charms many visitors and befuddles others. An outdoor fire escape makes sense, but an outdoor, wrought-iron flight of stairs that serves as a home’s sole means of egress in a city with an average yearly snowfall of 84 inches? People not from Montréal might see this as an inconvenience, if not a hazard. Like many things in the city, however, les escaliers evolved as a combination of practicality, creativity, and community planning.

Nineteenth-century law specified the amount of outdoor space required in front of every building. The mandate increased greenery in residential neighborhoods but reduced living space. Meanwhile, propelled by mass migration and record-high birth rates among Québécois, Montréal’s population nearly tripled from 1891 to 1921.

Some attribute the growth to la revanche du berceau , or “the revenge of the cradle,” a nationalist mission to fortify the population of the French-speaking province. Relocating staircases to the outside freed much-needed square footage inside the duplexes for expanding families.

Montréal prohibited new construction of external staircases in 1940 but lifted the ban in 1994 on streets where they already existed. Today, everything from classically ornate, winding stairways to minimalist, utilitarian designs adorn duplexes, bridging past and future in a city that boasts some of North America’s most diverse and imaginative architecture.

Register for the 2010 RI Convention in Montréal.