voussoir

noun

1. One of a series of wedge-shaped bricks or stones forming an arch or vault.

   “It is the voussoir depth in a real arch which enables the arch to carry wider ranges of loading; a large number of different idealized centre-line arches can be contained within a given practical profile. ...[T]his must be so, or no mediaeval bridge would have survived its decentering.”

   “Other areas have mortar joints missing, including between the voussoir blocks forming the edge of the arches on either side of the pier.”