A pier is usually built comparatively thin in the direction of the line of the bridge, because the forces tending to produce overturning in that direction are usually very small. When a series of stone arches are placed on piers, the thrusts of the two arches on each side of a pier nearly balance each other, and it is only necessary for the pier to be sufficiently rigid to withstand the effect of an eccentric loading on the arches; but if, by any accident or failure, one arch is destroyed, the thrust on such a pier is unbalanced and the pier will probably be overturned by the unbalanced thrust of the adjoining arch. The failure of that arch would similarly cause the failure of the succeeding pier and arch. On this account a very long series of arches usually includes an abutment pier for every fourth or fifth pier. An abutment pier is one which has sufficient thickness to withstand the thrust of an arch, even though it is not balanced by the thrust of an arch on the other side of the pier. Abutment piers are chiefly for arch bridges; but all piers should have sufficient rigidity in the direction of the line of the bridge so that any possible thrust which may come from the action of a truss of the bridge may be resisted, even if there is no counterbalancing thrust from an adjoining truss.