Centring, centre, centering, or center is a type of falsework: the temporary structure upon which the stones of an arch or vault are laid during construction. Once the arch is complete, it supports itself, but until the keystone is inserted, it has no strength and needs the centring to keep the voussoirs in their correct relative positions. A simple centering without a truss is called a common centering. The cross piece connecting centering frames are called a lag or bolst.