In a true arch (left) the stones form a circle that supports itself. A corbelled arch (right) is built by letting successive stones overhang slightly. The steps produced by the corbelled construction are then hewn away to produce a smooth profile.
The main teple complex of Altun Ha, a Maya city in Belize, contains a good example of corbelled arch technology in Maya architecture.
Photo: © Clark Anderson/Aquaimages, Creative Commons license (Wikipedia)