Original: The high walls and vaulted stone roofs were often supported by flying buttresses: half arches which transmit the outward thrust of the superstructure to supports or buttresses, often visible on the exterior of the building.
Simplified: The high walls and vaulted stone roofs were often supported by flying buttresses: half arches which transmit the outward thrust of the superstructure to supports or buttresses, usually on the outside of the building.