Keystone

Structurally and symbolically, the keystone functions as the central stone that binds and completes an arch or dome, with variations in shape—such as flat or multi-sided—reflecting differing ritual traditions, as a paper read before Quatuor Coronati Lodge notes. Within the emblem of the arch, it occupies the central position of a universal symbol that supports itself through a balance of opposing strains, representing the stability in tension by which the universe is maintained, as research presented at Quatuor Coronati Lodge explains. Early Masonic engravings featured the keystone prominently at the centre of such arches.

Although central to the imagery of the Royal Arch, the emblem also serves a foundational role in other Masonic degrees; in Mark Masonry, which is a distinct Order, Mark Masons' Hall adopts the symbol directly, describing itself as "The Keystone To Masonic Excellence".