Although the Romans are famous for their use of the arch, they did not invent the architectural element. It originated with the Etruscans, who inhabited regions of present-day Italy prior to the rise of the Romans. The Etruscans greatly influenced Roman architecture and many other aspects of Roman civilization.
The bricks used to create the arched portion of a Roman arch are called voussoirs. When properly aligned in the correct semi-circular arrangement, the voussoirs push downward and outward, holding the arch in place.
The voussoir located in the center and topmost section of a Roman arch is called the keystone. The keystone pushes outward on the other voussoirs, and it transfers the weight of the arch outward and downward toward the vertical supporting posts of the arch. It is often larger and decorated more ornately than the other voussoirs.
Springers are the lowest and bottommost voussoirs on each side of a Roman arch, and they mark the beginning of the spring line. The spring line is the point where the vertical support for the arch ends and the curve of the arch begins.
The rise of a Roman arch is the vertical distance between the height of the spring line and the height of the keystone. The span of the arch is the horizontal distance the arch spans, which is the distance between the two vertical support posts.
If a Roman arch is constructed properly, a cement or mortar is not necessary to hold the arch together. The pressure from each of the arch's stones pushing against one another is enough to maintain the structure's shape. Although mortar was not wholly necessary, the Romans often used a cementlike mixture of ash and lime to strengthen the construction of their arches as well as their buildings.
Framework was often used to outline the interior of a Roman arch to ensure that angles were correct and to take pressure off the voussoirs during construction. The framework was removed once the keystone was put in place and the pressure was balanced among the voussoirs.