At Edgar Kaufmann Sr.'s suggestion, where the windows meet the ragged stones walls, the glass runs directly into caulking channels, blurring the line between the exterior and interior space. Fallingwater, sometimes referred to as the Edgar J. Kaufmann Sr. Residence or just the Kaufmann Residence, located within a 5,100-acre nature reserve 50 miles southeast of Pittsburgh, was designed by Frank Lloyd Wright and built between 1936 and 1939. Built over a 30-foot flowing waterfall on Bear Run in the Mill Run section of Stewart Township, Fayette County, Pennsylvania, the house served as a vacation retreat for the Kaufmann family including patriarch, Edgar Kaufmann Sr., was a successful Pittsburgh businessman and president of Kaufmann's Department Store, and his son, Edgar Kaufmann, Jr., who studied architecture briefly under Wright. Wright collaborated with staff engineers Mendel Glickman and William Wesley Peters on the structural design, and assigned his apprentice, Robert Mosher, as his permanent on-site representative throughout construction. Despite frequent conflicts between Wright, Kaufmann, and the construction contractor, the home and guesthouse were finally constructed at a cost of $155,000. Quarrying of the Pottsville sandstone for the Fallingwater walls began the winter before the house's foundation was laid, in 1935-1936, under the direction of masonry contractor Norbert James Zeller. Zeller's men transported the stones via horse-drawn sled from the quarry, just up the hill. Frank Lloyd Wright specified that some of the stones should be as long as possible while others should be of various lengths. Fallingwater was designated a National Historic Landmark in 1966. It was listed among the Smithsonian's 28 Places to See Before You Die. In a 1991 poll of members of the American Institute of Architects (AIA), it was voted "the best all-time work of American architecture." In 2007, Fallingwater was ranked #29 on the AIA 150 America's Favorite Architecture list. National Register #74001781 (1974)