Such method of construction lessened the tendency of the arch or vaulting to exert a lateral strain or _thrust_ which occurs when the arch is composed of _voussoirs_ or, similarly, separate blocks of stone or brick are used in the vault. It tended to concentrate the strain on the vertical supports. Yet the Romans, though concentration of strains was a chief principle of their building, took no chances in the matter of stability and also distributed the strains. For example, the nave vaulting of a basilica would be reinforced by aisle vaulting, which was carried on walls that were either at right angles or parallel to the nave. But owing to the method of concrete construction and to the facility with which it could be employed, the Romans were able to erect vaults over buildings of complex plan and spaces of great size. The vaultings were of three kinds: 1. The _semicylindrical_ vault, called also the _wagon-headed_ vault or _barrel_ vault. 2. The _cross_ or _groined_ vault. 3. The _dome_ or _semidome_. The semicylindrical vault was a continuous arch spanning an oblong space, a corridor, and sometimes a curved passageway. The cross or groined vault was used over square spaces, its weight being carried at the four angles. It was formed by the intersection at right-angles of two semicylindrical vaults. When employed over long apartments or corridors, the ceiling was divided into a series of square compartments or _bays_, each covered by a cross-vault. Since the vaulting in each case was carried upon the corner supports, these became piers, and the wall spaces in between them, being thus relieved of the pressure of the vaulting, could be utilised for the openings of doors and windows. Moreover, a square space could be subdivided into bays, rendering it possible to vault a large area with no interruption to the floor-space except that of the piers. The dome was used for covering circular spaces, and when the space is small the covering is called a _cupola_ or little cup. Semidomes were employed over recesses. The finest existing example of a Roman dome is that of the =Pantheon=, which, however, affords an exception to the usual method of construction. For here, instead of being composed of concrete, thus forming a solid shell, the dome, so far as it has been examined, is found to be built of bricks, laid in almost horizontal courses. It is to be noted that the so-called “pendentive” dome, supported by arches over a space, _square in plan_, is not found in strictly Roman buildings and was a development of the Byzantine architects.