New York has its bridge-and-tunnel crowd, who must cross water to get to Manhattan. Sydney has its "M" people, who must take one of the M highways from the outer suburbs to get into the city. And Toronto has those who live in the ring of municipalities around the metropolis and are unified by – and named after – their area code: the 905ers.

At 2 a.m. on Oct. 2, 1993, Bell Canada flipped a switch that split the Greater Toronto Area in two. The telecom giant introduced an extra area code in the region – for the first time ever in Canada – to handle the soaring demand for new phone numbers brought on by the era's hottest technology: fax machines and pagers. But adding the 905 area code did much more than that: It created a sharp suburban-urban divide that has come to define the region 20 years later.

It's an oddity of the communications age that phone-company bureaucracy can delineate a political and emotional entity – and one that can never be replicated, now that the telecommunications industry has taken the area out of area codes. Any dialling prefix added today is an overlay, blanketing an entire region with a new code for new numbers rather than requiring some to change over. There were other area codes outside Toronto that split territory, but none that created a political identity akin to the 905: separating Greater Vancouver from Vancouver Island and most of B.C.'s mainland in 1996; dividing Montreal from its suburbs in 1998. The Edmonton-Calgary area-code split in 1999 proved to be the last one based on geography in Canada – and those two Alberta cities already had distinct, opposing identities.

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After that, the agency that distributes numbers switched to overlays; the new 647 code is sprinkled throughout the Greater Toronto Area, for instance. But the 416/905 division is a lasting legacy, creating a clear rift and, from that, a region. The implications have never been plainer, with an ongoing struggle between Toronto and its periphery on a host of planning issues. Even the proliferation of area-code-scrambling cellphones has failed to soften the designation. "The 905" became a catch-all for the cities and towns that ring Toronto, but also caught on among Torontonians as a pejorative – a shorthand for the trappings of suburban life.

"It's the stigma of mainstream culture," said Justin Noble, 30, who grew up in Brampton. "Costco and minivans kind of thing." Through his wife, who grew up in Toronto's core, he understands how "the 416" sees "the 905."

"For us, we like to think Brampton is almost Toronto," he said, "but for her, it's almost like Nunavut. Might as well be Russia."

The divide also represents a different attitude toward expansion. "The 416 is very much the mentality of how do we find a way to grow up," said political consultant Brian Kelcey, "whereas in the 905 it's a much more conventional growth that's focused on single-family homes and the aspirations of many new Canadians to have a home with a plot of land and room for a family to live and play."

To be taken seriously as "real partners" in transportation projects and funding models, these municipalities have had to create an informal collective, he says. "There's still a failure to treat a city like Mississauga or a city like Brampton … as the real, sizeable economic powerhouses they are," Mr. Kelcey said.

While the 905 area code established a region unified by the simple fact of not being Toronto, painting a large area with the same brush has been problematic. The mix of municipalities that formed the half-donut around Toronto had vastly different identities: some were auto manufacturing hubs, others were rural communities, others high-tech centres.

When the Toronto Board of Trade amended its name to the Toronto Region Board of Trade in January to include all the 905 municipalities under its umbrella, Mississauga Mayor Hazel McCallion was annoyed: Her city and several others in the region already had their own autonomous boards of trade, she argued.

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Few know that the GTA nearly escaped the fate of a deeply entrenched urban-suburban divide.

In the early 1990s, Bell Canada considered severing the region with one area code for east of Yonge Street and another for the west, explains Greg Duffy, who was a manager with Bell at the time. But Mr. Duffy's team canned the split marked by Canada's longest street due to a lack of faith in the public's geographic literacy.

"They don't necessarily know what east and west is," he said. "They don't know where Yonge Street is."

A GTA split down the middle might have been easier for Barbara Hall to govern. Ms. Hall, who was Toronto's mayor from 1994 to 1997, said the area code proved to be disruptive when she tried to collaborate with mayors in surrounding municipalities on such topics as economic development and transportation planning – a problem that has also plagued her successors.

"I was very focused on how to get the region that Toronto is a part of to see itself as and act as a region. [The area code] created a barrier to that," she said. An east-west split might have been less culturally divisive.

Kevin Menzel, 28, who grew up in Toronto, said he endured some ribbing when he moved to Mississauga, but was surprised by what he found. "There was more diversity than I was expecting in terms of types of neighbourhoods, density. It had been presented as fairly homogenous. I think [the differences] are overstated." He said he is puzzled by the staying power of the 416/905 divide, even with four new codes in the GTA and such a free flow of people – both commuters and transplants – between the two regions.

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Mr. Noble, who was in elementary school when the 905 was introduced, took pride in the distinction. In high school, he and a group of friends began calling themselves "Nine05" (there were nine of them and they would be graduating university in 2005). They even registered the domain nine05.com so they could have e-mail addresses that trumpeted which part of the GTA they were from.

"In Toronto, you might go to the park and hang out with your friends as a teenager," he said. "In Brampton, you drive to the 24-hour McDonald's or the 3 a.m. Burger King drive-through and you sit on the hood of your car and see who rolls up."