American Scientist

For millennia, structural arches have been assembled out of individual wedge-shaped stones known as voussoirs, with the last and topmost one to be put into place called the keystone. Since an arch must have all of its stones assembled properly before it can support itself, let alone the burden of a roadway built atop it, it has always been necessary to begin the construction process by erecting a timber scaffolding, commonly known as falsework or centering, whose convex upper surface matched the concave lower surface of the arch. The individual arch stones were then placed one by one on the falsework, working inward toward the center more or less symmetrically from each abutment against which the completed arch and its burden would finally push with full force. After the keystone was in place, the falsework was knocked out from under the stonework, leaving the completed arch spanning the gap between abutments.