voussoir

A voussoir is a wedge‑shaped block used to construct an arch or vault. Each piece is cut so that its inner face is thinner than its outer face, and its sides are angled to fit tightly against neighboring blocks. When arranged in a curved row, the voussoirs form a self‑supporting, load‑bearing ring. The central voussoir at the crown is typically the keystone, a special block that locks the arch together; in segmented arches, several top voussoirs can take on keystone‑like roles in distributing stresses.

Mechanics and geometry play a key role in the function of a voussoir arch. The wedge shape directs the weight of the structure into compressive forces that travel along lines toward the springing supports. As the blocks press against each other, friction and precise shaping prevent separation, allowing the arch to transfer loads to the abutments without the need for a continuous center support. Joints are usually tight and may be pointed with mortar in historical work; in some cases, the voussoirs are cut to fit with very small gaps or are dry‑laid.

Materials and construction methods vary. Traditional voussoirs are cut stone or brick segments, forming stone or brick arches and vaults in Roman and medieval architecture. In modern construction, precast concrete segments or brick modules are used to create segmented arches and vaults, offering consistency and rapid erection.

Etymology and usage: the term comes from French voûte, meaning vault, with voussoir meaning a wedge‑shaped block. The concept underpins many arch designs across cultures and periods.