Compound arch

A compound arch is an arch built using multiple independent sub-arches stacked vertically, with their arcs of voussoirs placed one on the top of the other. The goal of using a compound arch is usually to increase the overall strength and reliability (if only one order is fractured, the structure still stands). Each of these sub-arches, or "rings", of which the whole compound arch is composed, is called an arch order. In some compound orders their faces are in the same plane. But as a rule the orders are successively recessed, i.e. the innermost sub-arch, or order, is narrow, the next above it broader, the next is broader still, and so on.