The Probert Encyclopaedia of Architecture

ARCH

In architecture an arch is a usually curved member made up of separate wedge-shaped solids, with the joints between them disposed in the direction of the radii of the curve; they are used to support the wall or other weight above an opening. In this sense arches are segmental, round (i.e. semicircular), or pointed. A flat arch is a member constructed of stones cut into wedges or other shapes so as to support each other without rising in a curve. Scientifically considered, the arch is a means of spanning an opening by resolving vertical pressure into horizontal or diagonal thrust. The simplest form of an arch is two almost vertical supports lent inwards against one another.
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