Where marble is used, the peculiar fineness of its surface, upon which the bright Italian sun makes the smallest moulding effective, combined with the fact that the material, being costly, is often used in thin slabs, has given occasion to extreme flatness of treatment, and to the use of modes of enrichment which do not require much depth of material. Our illustration of a window from the Piazza S. Croce at Tivoli, shows these peculiarities extremely well (Fig. 56), and also illustrates the strong predilection which the Italian architects retained throughout the Gothic period for squareness and for horizontal lines. The whole ornamental treatment is here square; the window rests on a strongly-moulded horizontal sill, and is surrounded by flatly-carved enrichment, making a square panel of the entire feature. Even in the richly-decorated window (Fig. 57), which is in its pointed outline more truly Gothic than the Tivoli example, much of the same quality can be traced. The arch and jamb are richly moulded, but the whole mass of mouldings is flat, and the flat cuspings of the tracery, elaborately carved though it be, more resemble the cusps of early Western Gothic, executed at a time when tracery was beginning its career, than work belonging to the period of full maturity to which this feature, as a whole, undoubtedly belongs. Where marbles were plentiful enough to be built into the fabric, the national love of colour gave rise to the use of black and white--or sometimes red and white--alternate courses, already mentioned. The effect of this striped masonry may be partly judged of from the illustration of the cathedral at Siena (Fig. 52), where it is employed to a considerable extent. A finer method of surface decoration, less simple, however, and perhaps less frequently practised, was open to the Italian architect, in the use of panels of various coloured marbles. A beautiful example of the employment of this expedient exists in Giotto's campanile at Florence (Fig. 51). [Illustration: FIG. 57.--ITALIAN GOTHIC WINDOW, WITH TRACERY IN THE HEAD. (13TH CENTURY.)] The flatness of the roofs, which the Italians never abandoned, was always found difficult to reconcile with the Gothic tendency to height and steepness. In many cases, the sharp pitched gables which the buildings display, are only masks, and do not truly denote the pitch of the roofs behind them. In other instances the walls finish with a horizontal parapet, plain or ornamental, quite concealing the roof. In the roofs of their campaniles, however, the Gothic architects of Italy were usually happy; they almost always adopted a steep conical terminal, with or without pinnacles, which is very telling against the sky; even if its junction with the tower is at times clumsy.