The Monadnock Building (historically the Monadnock Block; pronounced /məˈnædnɒk/ mə-NAD-nok) is a skyscraper located at 53 West Jackson Boulevard in the south Loop area of Chicago, Illinois. The north half of the building was designed by the firm of Burnham & Root and built starting in 1891. The tallest commercial iron frame building with a load-bearing exterior masonry wall ever constructed, it employed the first portal system of wind bracing in America.