A voussoir is a type of brick or block used in arch construction that makes up the rounded portion at the top. Generally, a voussoir is wedge shaped or tapered in some way, with the narrower or thinner end pointing toward the bottom, or interior, of the arch and the wider or thicker end pointing upward, or towards the outside of the arch. Alternatively, regular rectangular bricks used in the same way may be called voussoirs.
The term voussoir comes is French, meaning "to turn or rotate." This refers to the ability of the blocks to turn, or transfer, the load on the arch from the blocks forming the curved section to the vertical supports. This load transference is the key principle behind all arches and is one of the most important architectural design elements in the buildings of many cultures and civilizations dating back to ancient times.
A typical arch is made up of vertical stacks of blocks, which may be free-standing or incorporated into a wall. Atop the vertical supports is a rounded section, which can vary in profile and can describe an arc or segment of a true circle or other shape, such as a parabola. The blocks or bricks at the top of the arch are the voussoirs. A voussoir is generally wedge shaped. Many arches are built with stone blocks, and the voussoirs may be placed with or without mortar. An arch built with voussoirs is very strong, capable of supporting great loads.
The principle behind an arch is that each individual voussoir, by virtue of its wedge shape and the rock or brick's strength under compression, beginning at the top of the arch, transfers the load above it in a radial fashion to the next voussoir in the series, resulting in the load being transferred to the vertical supports. This depends on a specific block, a voussoir called the keystone. The keystone is located at the center of the peak of an arch and provides the center point for the load transfer to either side of the arch as it descends to the vertical supports.
Cut stone blocks or cast bricks can be used as voussoirs, and arches using them may be constructed with or without mortar. While a basic voussoir has a simple wedge shape, variations on this shape are possible, used more in the interest of aesthetics than function. Blocks of alternating colors, intricate shapes or cuts, with varying angles are often employed. Many architectural styles have distinct types of voussoirs. Occasionally, arches constructed from single rows or multiple courses of rectangular bricks with wedge shaped layers of mortar are seen, and the bricks in these arches are also sometimes called voussoirs.