The red heraldic cross represents the "Christian motives and principles which governed the founders of the city," according to the city's official web site. The four floral emblems represent the four main European ethnic groups that were settled in the city in the 19th century. Clockwise from top left, they are:
Montreal's original coat of arms were designed in 1833 by its first mayor, Jacques Viger. They were similar to the current version with the difference that the red cross was a saltire and a beaver stood in the place of what is now a fleur-de-lys. The arms were revised in 1938 to which Montreal’s current flag is based.
Four years prior to the first raising of the current flag, King George V, the sovereign of Canada, celebrated his Silver Jubilee (25th year on the throne) on Monday May 6, 1935. An article in the Montreal Gazette from May 3, 1935 reported that the city’s Jubilee committee had discovered a rule whereby official coats of arms of British corporations could be interpreted into flag banners. Per the article, Montreal prepared to raise its own distinct flag for the first time ever on the King’s Jubilee day based on the original coat of arms by Jacques Viger.