At age 26, Burnham moved on to the Chicago offices of Carter, Drake and Wight, where he met future business partner John Wellborn Root, who was 21, four years younger than Burnham. The two became friends and then opened an architectural office together in 1873. Unlike his previous ventures, Burnham stuck to this one. Burnham and Root went on to become a very successful firm. Their first major commission came from John B. Sherman, the superintendent of the massive Union Stock Yards in Chicago, which provided the liveliehood – directly or indirectly – for one-fifth of the city's population. Sherman hired the firm to build for him a mansion on Prairie Avenue at Twenty-first Street among the mansions of Chicago's other merchant barons. Root made the initial design. Burnham refined it and supervised the construction. It was on the construction site that he met Sherman's daughter, Margaret, whom Burnham would marry in 1876 after a short courtship. Sherman would commission other projects from Burnham and Root, including the Stone Gate, an entry portal to the stockyards, which became a Chicago landmark.