More important, he found a way to manufacture these handcrafted pieces himself. In 1953, he and his identical twin, Fulgenzio, founded a company, Tecno, for which they built a sprawling factory in 1962 on the site of what was once their father’s wood shop. By controlling every step of fabrication and distribution, and incorporating a unified graphic identity long before such a concept was de rigueur (its curvilinear “T” logo became ubiquitous in magazine advertisements), the brothers came to dominate in 1970s-era office design. Osvaldo enlisted prominent architects, including Norman Foster and Gae Aulenti, to design furniture collections and component systems, which were displayed in more than 40 showrooms, from Paris and Chicago to Tokyo and Buenos Aires — the first such scale operation out of Italy. Osvaldo’s daughter Valeria and her husband, Marco Fantoni (parents of Tommaso and Giacomo), eventually joined the company, as did Fulgenzio’s son, Paolo.

The clan was always working, so the boundaries between the factory — six stories of poured and precast concrete — and the 8,000-square-foot palazzo next door were porous. The contrast of the structures, and the ease with which the family traversed the eras, was a living illustration of how artisanal craftsmanship could coexist with mass production. “They would all be in the factory in the morning, and then stroll back here, with clients and collaborators, for lunch,” says Tommaso, standing in the dining room of the house beside a vintage green Brazilian onyx table surrounded by upholstered chairs of Borsani’s design. “It was really seamless for so many years.”

AFTER OSVALDO’S DEATH, Valeria and Marco took over design duties at Tecno, but once Fulgenzio, who oversaw the financial side, died in 1991, the two branches of the family amicably divided the assets. The Fantonis retained the rights to the archival furniture designs; Fulgenzio’s heirs kept the interest in Tecno (his grandson, Federico Borsani, remains an executive). Its Varedo factory was converted into apartments in the early aughts. But the family was united in maintaining Villa Borsani as its creator had intended. Still occasionally occupied by relatives, it also contains Osvaldo’s vast archive, including thousands of the architect’s watercolor renderings of interiors and products.