If this be unaisled and unvaulted, it is a very simple affair. Windows at a reasonable height, dealt with agreeably to the architectural grade of the building,--probably a base-course, a string beneath the cills, and possibly pilaster buttresses between the windows, and a corbel-table uniting the same under the eaves. If vaulted with a wagon-vault (as, for instance, St. Cormac’s Chapel, at Cashel),[23] the walls must be higher, and, it may be, the dead space which this occasions externally may be decorated by arcading. If, however, it is groined, the difficulty disappears. Again, an aisled but unvaulted nave is of simple construction, but if the aisle be vaulted (unless, indeed, it be a mere demi-vault, which in this country is very rare), a greater complication is brought into existence. The groining requires that the aisle wall shall be fully as high as the crown of the arches between the aisles and the nave; and, as the aisle roof demands some reasonable height, it follows that there must be a considerable space of wall above the arches. This may be dealt with in several ways. If the nave be unvaulted, it is a blank space, or may be pierced by an arcade or other openings. If the nave is groined without a clerestory, the space is partly occupied by the springers of the vaulting, and the intervals may be pierced. If there is a clerestory, the space becomes what we call (though erroneously), a _triforium_; or (whether there be a clerestory or not), it may be made more of and utilised by raising the aisle walls sufficiently to convert it into a second storey or gallery to the aisle (Fig. 241). [Illustration: Fig. 241.]