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Ever since the Flatiron Building was erected at the foot of Madison Square Park in Manhattan in 1902, it has brimmed with tenants, most of which were small businesses.

Songwriters tickled piano keys for sheet-music publishers. Dentists buzzed their drills. At one company, milliners made fezzes for the Masons.

Things began to change after 1959, when St. Martin’s Press moved in and started to expand, snagging offices when other tenants departed. By 2004, its parent company, Macmillan Publishers, had taken over all 21 office floors of the building.