The second distinctive principle of Gothic architecture was that of _balanced thrusts_. In Roman buildings the thrust of the vaulting was resisted wholly by the inertia of mass in the abutments. In Gothic architecture thrusts were as far as possible resisted by counter-thrusts, and the final resultant pressure was transmitted by flying half-arches across the intervening portions of the structure to external buttresses placed at convenient points. This combination of flying half-arches and buttresses is called the _flying-buttress_ (Fig. 107). It reached its highest development in the thirteenth and fourteenth centuries in the cathedrals of central and northern France. +RIBBED VAULTING.+ These two principles formed the structural basis of the Gothic styles. Their application led to the introduction of two other elements, second only to them in importance, _ribbed vaulting_ and the _pointed arch_. [Illustration: FIG. 108.--RIBBED VAULT, ENGLISH TYPE, WITH DIVIDED GROIN-RIBS AND RIDGE-RIBS.] The first of these resulted from the effort to overcome certain practical difficulties encountered in the building of large groined vaults. As ordinarily constructed, a groined vault like that in Fig. 47, must be built as one structure, upon wooden centrings supporting its whole extent. The Romanesque architects conceived the idea of constructing an independent skeleton of ribs. Two of these were built against the wall (_wall-ribs_), two across the nave (transverse ribs); and two others were made to coincide with the groins (Figs. 98, 108). The _groin-ribs_, intersecting at the centre of the vault, divided each bay into four triangular portions, or _compartments_, each of which was really an independent vault which could be separately constructed upon light centrings supported by the groin-ribs themselves. This principle, though identical in essence with the Roman system of brick skeleton-ribs for concrete vaults, was, in application and detail, superior to it, both from the scientific and artistic point of view. The ribs, richly moulded, became, in the hands of the Gothic architects, important decorative features. In practice the builder gave to each set of ribs independently the curvature he desired. The vaulting-surfaces were then easily twisted or warped so as to fit the various ribs, which, being already in place, served as guides for their construction. [Illustration: FIG. 109.--PENETRATIONS AND INTERSECTIONS OF VAULTS. _a, a_, _Penetrations by small semi-circular vaults sprung from same level_. b, _Intersection by small semi-circular vault sprung from higher level; groins form wavy lines_. c, _Intersection by narrow pointed vault sprung from same level; groins are plane curves_.]