Voussoir

Voussoir

a name adopted from the French for the wedge-shaped stones (or other material) with which an arch is constructed, as marked in the engraving a, a, a; the upper one, namely, b, at the crown of the arch, is termed the key- stone. The lowest voussoir, or that which is placed immediately above the impost, is termed the spring stone, or springer. —Parker, Gloss. of Architect. s.v.