Dulles's iconic main terminal houses ticketing on the upper level, baggage claim and U.S. Customs and Border Protection on the lower level, and annexes for the International Arrivals Building for international passenger processing, as well as the Z gates (used by Air Canada, Frontier and United Express), various information kiosks and other support facilities. The main terminal was recognized by the American Institute of Architects in 1966 for its design concept; its roof is a suspended catenary providing a wide enclosed area unimpeded by any columns.