Voussoirs are wedged-shaped units, usually made from stone, that are used when building an arch. Often, each stone that comprises an arch is referred to a as a voussoir, though there are also two distinctive types of voussoir stones: keystones and springers. Each type of stone has a different purpose pertaining to the actual structure of an arch.
A keystone is a center stone that is placed within the apex of an arch. This stone tends to be larger than all the rest, and it is usually elaborately decorated. The purpose of a keystone is to lock all the other stones into their proper positions. Throughout history, keystones were regarded as important stones that were aesthetically pleasing and structurally crucial. Today, some believe that keystones are not as important as springer stones, though this is debatable.
Springer stones are the lowest stones in an archway. These stones are often placed on either side of the arch towards the bottom of the arch itself. Since these stones support an entire arch, they are vital to any sound arch construction. Both the keystone and the springer are considered voussoirs, though the various bricks that make up an archway can also fall under this category.
Arches that are built using the voussoir system are often long-lasting. Each stone within this type of structure helps to distribute the overall weight of an arch evenly. Essentially, each stone turns against the next one causing durability and strength. While these stones are not as popular as they once were, voussoirs are still used in some parts of the world.
Islamic and Moorish architecture relied heavily upon voussoirs, which are often red and white in color. Some of the finest voussoirs can be found inside of mosques. Examples of archways that use the voussoir technique can also be found in various countries including Italy, Greece, and Turkey. Technically, voussoirs belong in the Medieval architecture category, though these stones were used for centuries following the Medieval time period.
Modern arch architecture within North America tends to favor pre-cast stones in places of the voussoir. In fact, many ancient archways are refaced using pre-cast stones. Architects tend to agree that these units are sturdier than voussoirs, though the appearance of a pre-cast archway is not the same as one created using the voussoir technique. Conservation efforts around the globe aim to replace impressive Medieval archway stones with pre-cast stones.