I have preferred to call the arch mid-cusped with respect to its side piece than with respect to its own curve, because the most beautiful Gothic arches in the world, those of the Lombard Gothic, have, in all the instances I have examined, a form more or less approximating to this mid-cusped one at _i_ (Plate III.), but having the curvature of the cusp carried up into the keystone, as we shall see presently: where, however, the arch is built of many voussoirs, a mid-cusped arch will mean one which has the point of the cusp midway between its own base and apex. The Gothic arch of Venice is almost invariably up-cusped, as at _k_. The reader may note that, in both down-cusped and up-cusped arches, the piece of stone, added to form the cusp, is of the shape of a scymitar, held down in the one case and up in the other. § XVIII. Now, in the arches _h_, _i_, _k_, a slight modification has been made in the form of the central piece, in order that it may continue the curve of the cusp. This modification is not to be given to it in practice without considerable nicety of workmanship; and some curious results took place in Venice from this difficulty. At _l_ (Plate III.) is the shape of the Venetian side stone, with its cusp detached from the arch. Nothing can possibly be better or more graceful, or have the weight better disposed in order to cause it to nod forwards against the keystone, as above explained, Ch. X. § II., where I developed the whole system of the arch from three pieces, in order that the reader might now clearly see the use of the weight of the cusp. Now a Venetian Gothic palace has usually at least three stories; with perhaps ten or twelve windows in each story, and this on two or three of its sides, requiring altogether some hundred to a hundred and fifty side pieces. I have no doubt, from observation of the way the windows are set together, that the side pieces were carved in pairs, like hooks, of which the keystones were to be the eyes; that these side pieces were ordered by the architect in the gross, and were used by him sometimes for wider, sometimes for narrower windows; bevelling the two ends as required, fitting in keystones as he best could, and now and then varying the arrangement by turning the side pieces _upside down_. There were various conveniences in this way of working, one of the principal being that the side pieces with their cusps were always cut to their complete form, and that no part of the cusp was carried out into the keystone, which followed the curve of the outer arch itself. The ornaments of the cusp might thus be worked without any troublesome reference to the rest of the arch.