At S. Sophia there was doubtless a large use made of temporary ties of this kind during the construction. In many places at the springing of the gynaeceum vaults the ends of such provisional ties, which have been sawn away, appear. Besides these there is a series of wood beams which from the first were intended to be permanent, for they are richly carved (C in Fig. 45); these are shown by double lines on the right-hand side of Figs. 5 and 6, the single lines showing the iron ties. These carved beams, as Choisy points out, are struts rather than ties. If we take one of the columns standing in an angle in the aisles, an impost of marble connects it with the wall to which it is nearest, and a carved wood beam forms a strut to the other wall. The beam across the central bay of secondary order (Fig. 5) forms a rigid strut to the two wider arches (see Fig. 38, where, however, by oversight the beam has been omitted; it is at the springing of narrow arch high above iron tie). Choisy asserts that “the architect intended to preserve only the struts, all the ties subject to extension were removed, but their suppression was disastrous, and they had hastily to replace them by bars of iron which were fixed with difficulty.” We do not know what reason Choisy had for supposing the system of iron ties to be an afterthought, unless it is because in some cases they appear directly above the ends of the removed wooden ties. Now we believe they occur equally above the carved beams in the openings from the gallery to the nave, and there is no sign of wood ties having been removed from the ground-floor vaults, where the iron bars fulfil such an important function. It is certain that the iron bars to all the nave arches are original, for the marble casing shows no sign of alteration, and they are evidently threaded continuously through the imposts. The important iron ties across the aisles are shown in Fig. 45: _d_ is the attachment to the column of great order, _e_ to impost of secondary order behind it, _f_ is a king rod. Across the west gallery the span is lessened by stone corbels beneath the ties _g_. With a view of binding the vaults and walls together into a homogeneous mass, the arched vaulting of the interior was carried through the thickness of the walls: in some cases these arches were left open, to be afterwards filled with a screen of windows. The walling of the sides of the church is built independently of the great piers, as straight joints on the exterior show, and Choisy remarks that the independence of masonry unequally charged was a leading idea in Byzantine construction; indeed it is obviously necessary where the quantity of mortar is so great that the brick at times becomes secondary to the joints. [Illustration: FIG. 45.--Methods of Chainage.]