BUTRESSES

Buttresses in architecture, is the exterior support, usually of masonry, projecting from the face of a wall and serving either to strengthen it or to resist the side thrust created by the load on an arch or a roof. In addition to their practical functions, buttresses can be decorative, both in their own right and from the designs carved or constructed into them. There are various types of buttresses such as the flying buttresses,  pier or tower buttresses, hanging buttresses and corner buttresses.

Picture taken from : http://www.americansinfrance.net/Attractions/Chartres-Cathedral-Flying-Buttress.cfm

Acknowledgement: http://www.britannica.com/EBchecked/topic/86773/buttress