In most Norman buildings we find very excellent masonry and massive construction. The exteriors of west fronts, transepts, and towers show great skill and care in their composition, the openings being always well grouped, and contrasted with plain wall-spaces; and a keen sense of proportion is perceptible. The Norman architects had at command a rich, if perhaps a rather rude, ornamentation, which they generally confined to individual features, especially doorways; on these they lavished mouldings and sculpture, the elaboration of which was set off by the plainness of the general structure. In the interior of the churches we usually meet with piers of massive proportion, sometimes round, sometimes octagonal, sometimes rectangular, and a shaft is sometimes carried up the face of the piers; as, for example, in Peterborough Cathedral (Fig. 173). The capitals of the columns and piers have a square abacus, and, generally speaking, are of the cushion-shaped sort, commonly known as basket-capitals, and are profusely carved. The larger churches have the nave roofed with a timber roof, and at Peterborough there is a wooden ceiling; in these cases the aisles only are vaulted, but in some small churches the whole building has been so covered. Buttresses are seldom required, owing to the great mass of the walls; when employed they have a very slight projection, but the same strips or pilasters which are used in German Romanesque occur here also. Low towers were common, and have been not unfrequently preserved in cases where the rest of the building has been removed. As the style advanced, the proportions of arcades became more lofty, and shafts became more slender, decorative arcades (Fig. 174) became more common, and in these and many other changes the approaching transition to Gothic may be easily detected. We have already alluded to the many Norman doorways remaining in parish churches of which all other parts have been rebuilt. These doorways are generally very rich; they possess a series of mouldings sometimes springing from shafts, sometimes running not only round the arched head, but also up the jambs of the opening; and each moulding is richly carved, very often with a repetition of the same ornament on each voussoir of the arch. Occasionally, but not frequently, large portions of wall-surface are covered by a diaper; that is to say, an ornament constantly repeated so as to produce a general sense of enrichment. [Illustration: FIG. 173.--NAVE ARCADE, PETERBOROUGH CATHEDRAL.] [Illustration: FIG. 174.--DECORATIVE ARCADE FROM CANTERBURY CATHEDRAL.]