=South Italy.=--The most important SOUTHERN examples are found in Sicily, which in the tenth century was overrun by the Saracens, who in the following century were routed by the Normans. Consequently, the Saracenic influence is mingled with the Byzantine in the =Cathedral of Monreale=, near =Palermo=. The plan is basilican, with apses at the eastern ends of the nave and aisles. The choir is raised. The arches of the nave are pointed but not recessed, and are supported on columns, with Byzantine capitals. The aisle walls have a dado of white marble, twelve feet high, inlaid with borders, composed of porphyry, while the arches and clerestory of the nave are embellished with mosaics of biblical subjects, framed in arabesque borders. Of a sombre richness of colour, they display the Byzantine characteristic of severity of design, and impart to the interior a solemn grandeur. =North Italy.=--It is in NORTHERN Italy, particularly in the Lombard churches, that the constructional development is most marked. For, while the plan remained basilican, only occasionally showing well-defined transepts, the architects devoted their energies to the problem of vaulting. A notable instance is =San Ambrogio, Milan=, which is an early example of the use of ribs in vaulting. The original church, erected in the ninth century, had wooden roofs; but in the rebuilding the nave was divided into four square bays, and immense piers were constructed to carry the diagonal, transverse, and longitudinal ribs.[6] Of corresponding massiveness are the transverse ribs, while to support the strain on the longitudinal ribs intermediate piers were introduced with an upper and a lower tier of double arches. These open into the two stories of the groin-vaulted aisles, which are given this treatment in order to act as buttresses to the thrust of the nave vaults. This compelled the omission of clerestory windows, thus adding to the sombreness of effect. Indeed the whole suggestion is one of ponderousness. It is the work of men experimenting with a new method of construction and intent for the present on achieving stability. The combination of the latter with dignity of height and the grace of lightness was yet to be developed in the Gothic treatment of the ribs. The west end is approached by a narthex, opening into an arcaded atrium. In the external decoration of the triple apse of the east end appears the rudimentary principle of the open arcade. The walls above the semi-dome and beneath the wooden exterior roof are crowned with a cornice, composed of arches supported upon corbels, the space between each being penetrated with a niche. This produces a series of deep shadows, in contrast with which the actual construction of the corbels assumes a lightness of effect. It was the preliminary step to the substitution of small detached columns for the corbels and the development of external arcading.