Early Gothic Flying Buttress
A flying buttress, or arc-boutant, is a specific type of buttress usually found on a religious building such as a cathedral. They are used to transmit the horizontal thrust of a vault across an intervening space (which might be an aisle, chapel or cloister), to a buttress outside the building. The employment of the flying buttress means that the load bearing walls can contain cut-outs, such as for large windows, that would otherwise seriously weaken them. Flying buttresses are often found in Gothic architecture.
A. D. F. Hamlin College Histories of Art History of Architecture (New York, NY: Longmans, Green, and Co., 1915)
Courtesy the private collection of Roy Winkelman