In 1931, Juan O'Gorman built a house of exposed concrete, industrial sash windows and a cactus fence for a painter who could barely pay for it. The commission came from Diego Rivera. The result — a cobalt-blue block and a red-and-white block joined by a rooftop bridge, at Diego Rivera 2 in Col. San Ángel Inn — is widely held to be the first functionalist building in Latin America. O'Gorman was twenty-six. He had read Le Corbusier and decided that ornament was a form of dishonesty. That decision set the terms for everything a modern architecture tour mexico city follows today. Within two decades, Mexican architects were testing whether a European grammar could carry Mexican weight: volcanic rock, folk colour, pre-Hispanic geometry, murals welded to structural surfaces. Luis Barragán answered with the pink and ochre walls of Casa Luis Barragán in Tacubaya. O'Gorman himself answered twice, returning in 1952 to sheathe the Central Library at Ciudad Universitaria in four façades of natural stone mosaic — ten million pieces, no pigment, every tone quarried. The studio-house survives because the state intervened. Rivera lived and worked there until 1940; Frida Kahlo occupied the adjoining block. The buildings passed to the nation in 1986 and opened as a museum under INBAL, with the painter's studio preserved close to its working state: easels, Judas figures in papier-mâché, pre-Columbian idols ranked along the shelves, north light falling through the sawtooth glazing. Admission is 50 MXN. Why it still matters is a question of lineage rather than nostalgia. The functionalist experiment in San Ángel produced the Ciudad Universitaria campus, the Torre Latinoamericana's seismic engineering, the brutalist mass of the Museo Nacional de Antropología, and the concrete umbrellas of Félix Candela's hyperbolic paraboloids. Contemporary practices in Polanco and Roma still cite the same vocabulary — raw finish, hard shadow, saturated planes. Guided architecture walking tours of the capital read the city as a continuous argument about how a modern nation should look. The canon is unusually legible on foot. Walls are low, streets are shaded, distances are short, and the shifts in register from 1931 to the 1970s occur within a few blocks. Visitors on a modern architecture tour mexico city encounter that argument at its source: two small houses in Álvaro Obregón where concrete, colour and painting were first asked to work as one thing.