Greenwald and Mies' concept to develop an apartment scheme with so much glass proved controversial. Finally, in Spring 1947, Greenwald was able to secure a mortgage with the Trust Company of Chicago. The Peter Hamlin Construction Company was selected as the general contractor and construction began that Fall. The project began to attract interest when a model of the final version and a drawing of the steel-and-glass version were put on display for a Mies showcase at the Museum of Modern Art. By the time the building was completed in 1949, all of the apartment units were sold. The Promontory Apartments were the first Mies building to have an exposed skeleton structure, an architectural theme that would dominate the latter portion of his career. Mies first proposed an external skeleton for an office building in 1923, but abandoned the concept until the late 1930s. His Mechanical Engineering Research building on the Illinois Institute of Technology campus featured an exposed concrete frame, but it was infilled with brick spandrel walls and windows and was not constructed in accordance with Mies' design. However, the building design is clearly a precursor to the Promontory Apartments. The Promontory Apartments were an important point in Mies' career. The building was well received in the United States and abroad and further cemented his reputation as a leading architect. The building's success would inspire Greenwald to commission Mies to design more than ten additional apartment projects. Mies was also encouraged by the reception and abandoned the classic elements such as the traditional floor plan for more experimental steel-and-glass design. The building and its display at the Museum of Modern Art encouraged Phyllis and Samuel Bronfman to commission Mies to design the Seagram Building in New York City. In the mid-1960s, the Promontory Apartments Trust contacted Mies about alterations to the ground floor as well as through-wall air conditioners for the spandrel walls. Mies offered his input on these changes, which were put into place c. 1965-66. On November 21, 1996, the building was recognized by the National Park Service with a listing on the National Register of Historic Places. The Promontory Apartments are adjacent to the 1927 Flamingo-on-the-Lake Apartments, also listed on the National Register. The building is designed as a Double T with the horizontal cross-bars joined; the stems of the T's form wings to the rear. Each T is its own building with separate addresses, elevators, and stairways.