Between the fragile, heritage, Second Empire styled facades of a former YMCA that is to be conserved (built in 1879), and the immensity of a polyvalent theater (625 seats) to be arranged architecturally on the cutting edge of scenic technology, it was necessary to create space within an enclosed site, in order to allow these two architectural and functional entities to exist. The decision to cut diagonally into the existing conditions uncompromisingly assures the materialization of the new theater and allows for the integration of a new semi-public space into the geometry of the urban tissue. A triangular void, which raises the entire height of the old building, acts as a space of light and life, as a place of public dispersion linked to the Place d’Youville (the cultural intersection of the city) , and allows the two entities, heritage building and theater, to fully co-exist. At the summit of this breach emerges a glass volume, in the background of the former YMCA. The volume of faceted glass, treated with an upward gradual translucence, signals the new cultural place and, at its heart, acts as an urban lantern during the evening and as an atrium during the day. Bridging the two parts of the L-shaped Place d’Youville, the crystalline prism enhances the two entries which lead to the center of the project. On Saint-Jean Street, it articulates itself in facets above the main entry, taking the place of the missing portion of the original building of the former YMCA . On Glacis Street, it envelops the theater’s new concrete volume, forming a large marquee which advances above a second entrance. Le Diamant, although resolutely contemporary as a whole, not only respects its existing heritage but also its phantoms.