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.