The most common characteristic by which the NORMAN style is distinguished, is the semicircular or segmental arch, though this is to be met with also in the rare specimens of Anglo-Saxon masonry; but the Norman arches were more scientifically constructed: in their early state, indeed, quite plain, but generally concentric, or one arch receding within another, and in an advanced stage they were frequently ornamented with zig-zag and other mouldings. A variety of mouldings were also used in the decoration of the Norman portals or doorways, which were besides often enriched with a profusion of sculptured ornament. The Norman churches appear to have much excelled in size the lowly structures of the Saxons, and the cathedral and conventual churches were frequently carried to the height of three tiers or rows of arches, one above another; blank arcades were also used to ornament the walls. [Illustration: Norman Arcade, St. Aldgate, Oxford.] The Norman style, in which an innumerable number of churches and monastic edifices were originally built or entirely reconstructed, continued without any striking alteration till about the latter part of the twelfth century, when a singular change began to take place: this was no other than the introduction of the pointed arch, the origin of which has never yet been satisfactorily explained, or the precise period clearly ascertained in which it first appeared; but as the lightness and simplicity of design to which the Early Pointed style was found to be afterwards convertible was in its incipient state unknown, it retained to the close of the twelfth century the heavy concomitants of the semicircular arch, with which indeed it was often intermixed: and from such intermixture it may be designated the SEMI or MIXED NORMAN. When the original Norman style of building was first broken through, by the introduction of the pointed arch, which was often formed by the intersection of semicircular arches, the facing of it, or architrave, was often ornamented with the zig-zag, billet, and other mouldings, in the same manner as the Norman semicircular arches: it also rested on round massive piers, and still retained many other features of Norman architecture. But from the time of its introduction to the close of the twelfth century, the pointed arch was gradually struggling with the semicircular arch for the mastery, and with success; for from the commencement of the thirteenth century, as nearly as can be ascertained, the style of building with semicircular arches was, with very few exceptions, altogether discarded, and superseded by its more elegant rival. [Illustration: Canterbury Cathedral.]