The Square Mile and also known as the Golden Square Mile (officially in French: Le Mille Carré and also known as Mille carré doré) is the nostalgic name given to an urban neighbourhood developed principally between 1850 and 1930 at the foot of Mount Royal, in the west-central section of downtown Montreal, Quebec, Canada. The name "Square Mile" has been used to refer to the area since the 1930s; prior to that, the neighborhood was known as 'New Town' or 'Uptown'. The addition of 'Golden' was coined by Montreal journalist Charlie Lazarus, and the name has connections to contemporary real-estate developments, as the historical delimitations of the Golden Square Mile overlap with Montreal's contemporary central business district.

From the 1790s, the business leaders of Montreal looked beyond Old Montreal for spacious sites upon which to build their country homes. They developed the farmland of the slopes of Mount Royal north of Sherbrooke Street, creating a neighborhood notorious for its grandeur and architectural audacity. At the Square Mile's peak (1850-1930), its residents included the owners and operators of the majority of Canadian rail, shipping, timber, mining, fur and banking industries. From about 1870 to 1900, 70% of all wealth in Canada was held by this small group of approximately fifty men.

By the 1930s, multiple factors led to the neighborhood's decline, including the Great Depression, the dawn of the automobile, the demand for more heat-efficient houses, and the younger generations of the families that had built these homes largely moved to Westmount. During the Quiet Revolution, some of the businesses created in Montreal, on whose fortunes the Square Mile had been built, moved to Toronto. In this period, the Square Mile evolved to gradually become the central business district, and many of grand houses were demolished. The face of the Square Mile was altered, leading to the formation of Heritage Montreal to preserve architecture in the city.

By 1983, only 30% of the mansions in the northern half of the Square Mile had survived demolition; and only 5% survived south of Sherbrooke Street. Many of the remaining mansions, such as the James Ross House, today known as Chancellor Day Hall, are today owned by McGill University. Nevertheless, the mansions of the Golden Square Mile represent a prosperous period during which Montreal was the cultural and financial capital of Canada.

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