He says farther on:--“We may therefore call the said vault, rib, and shaft; the rere-vault, rere-rib, and rere-shaft of the window.” He also remarks that, “in the thick walls of Mediæval structure, the tracery and its glazing are commonly placed much nearer to the outer surface of the wall than to the inner.” This last observation calls our attention to a great and important distinction by which nearly all Mediæval windows may be classified--viz., those which have their glass-plane at or near the mid-thickness of the wall, and those which, as the Professor says, have it “much nearer to the outer surface than to the inner.” This distinction was, as I have shown, of early date; being in its earlier ages rather distinctive of “Saxon” from Norman windows. The class, however, in which the glass was nearer the outer than the inner side had, up to about the year 1200, its inner arch concentric with its outer one; but the invention of the rere-arch and its accompaniments obviated this, and established a hard and obvious distinction between these two great classes of windows. The custom of sometimes placing the glass at the mid-thickness of the wall was in no degree given up, but, on the contrary, was continued through all styles; but, when adopted, the older system of making the inner concentric with the outer arch was nearly always continued, marking more distinctly the great difference between the two classes of window. The choice between them became a mere matter of taste and of outlay; all styles acknowledging both as equally admissible and correct. The two systems may be distinguished as _rere-arch_ windows and _through-arch_ windows--_i.e._, those in which the inner is distinct from the outer arch, and those in which the same arch runs _through_ the wall, showing itself more or less similarly on its outer and inner faces. [Illustration: Fig. 165.--Broughton Church, Oxfordshire.] [Illustration: Fig. 166.--Christchurch, Hants. North Transept.]