A vault is a structural element in building construction that consists of an arrangement of arches, commonly forming a ceiling or roof. When two barrel vaults crossed at right angles, they produced a groin vault, which, when repeated in sequence, could span rectangular expanses of infinite length. The Romans were the first to use this type of structure for large-scale buildings such as theaters and arenas. In architecture, a groined vault is a framework of intersecting ribs or posts from which spring transverse members called "transoms" or "trigues." The ribs may be curved or straight, but are most often cylindrical. They are placed close together with their open ends facing inward, so that the space within them is hollow and can be used for storage or as a viewing gallery. The truss design shown here can be used instead of individual ribs if desired. The word "vault" is also applied to similar structures without transverse support beams: a half-groined vault has its ribs set half way between floor and ceiling; a fully groined vault has its ribs all the way to the top or bottom. Half-groined vaults were common in ancient Rome where they were used for cisterns and other water-storage projects because they were easier to build than full-height vaults. The earliest known use of the term "vault" in reference to a metal container dates back to 1421.