The simplest kind of vault is the barrel vault (also called a wagon or tunnel vault), which is generally semicircular in shape. The barrel vault is a continuous arch, the length being greater than its diameter. As in building an arch, a temporary support is needed while rings of voussoirs are constructed and the rings placed in position. Until the topmost voussoir, the keystone, is positioned, the vault is not self-supporting. Where timber is easily obtained, this temporary support is provided by centering consisting of a framed truss with a semicircular or segmental head, which supports the voussoirs until the ring of the whole arch is completed. With a barrel vault, the centering can then be shifted on to support the next rings.
Amongst the earliest known examples of any form of vaulting is to be found in the neolithic village of Khirokitia on Cyprus. Dating from ca. 6000 BCE, the circular buildings supported beehive shaped corbel domed vaults of unfired mud-bricks and also represent the first evidence for settlements with an upper floor. Similar Beehive tombs, called tholoi, exist in Crete and Northern Iraq. Their construction differs from that at Khirokitia in that most appear partially buried and make provision for a dromos entry.
The inclusion of domes, however, represents a wider sense of the word vault. The distinction between the two is that a vault is essentially an arch which is extruded into the third dimension, whereas a dome is an arch revolved around its vertical axis.
Pitched-brick vaults are named for their construction, the bricks are installed vertically (not radially) and are leaning (pitched) at an angle: This allows their construction to be completed without the use of centering. Examples have been found in archaeological excavations in Mesopotamia dating to the 2nd and 3rd millennium BC which were set in gypsum mortar.
A barrel vault is the simplest form of a vault and resembles a barrel or tunnel cut lengthwise in half. The effect is that of a structure composed of continuous semicircular or pointed sections.
The earliest barrel vaults in ancient Egypt are thought to be those in the granaries built by the 19th dynasty Pharaoh Ramesses II, the ruins of which are behind the Ramesseum, at Thebes. The span was 12 feet (3.7 m) and the lower part of the arch was built in horizontal courses, up to about one-third of the height, and the rings above were inclined back at a slight angle, so that the bricks of each ring, laid flatwise, adhered till the ring was completed, no centering of any kind being required; the vault thus formed was elliptic in section, arising from the method of its construction. A similar system of construction was employed for the vault over the great hall at Ctesiphon, where the material employed was fired bricks or tiles of great dimensions, cemented with mortar; but the span was close upon 83 feet (25 m), and the thickness of the vault was nearly 5 feet (1.5 m) at the top, there being four rings of brickwork.