In Byzantine architecture, a vault formed by extending a semicircular arch along its length is a barrel vault.
A barrel vault creates a continuous, tunnel-like roof over a rectangular or elongated space. Its curved surface transfers weight mainly toward the supporting walls, so Byzantine builders often combined it with thick masonry, transverse arches, and other vaults.
Byzantine churches are especially associated with domes, pendentives, and centralized plans, but barrel vaults were also important. They could cover side aisles, galleries, entrance areas, and supporting spaces around a central dome. Large churches frequently combined several structural forms rather than relying on one vault type.
A barrel vault differs from a groin vault, which results when two barrel vaults intersect at right angles. It also differs from a dome, whose curved surface rises around a central point or axis. Roman construction provided major precedents for the technique, which Byzantine architects adapted to complex church layouts.