The pillars of the nave also present problems of some difficulty. They are alternately circular and octagonal, and the masonry of six of them points with something like certainty to a date considerably earlier than the twelfth century restoration. In the four western pillars the stones are a good deal smaller than those in the two octagonal ones of the next bay: this makes it highly probable that they are of earlier date than the octagonal pillars, which are certainly Norman of the period of the restoration c. 1160. Mr. Harrison believes there is also considerable evidence that the two cylindrical pillars were reduced in girth in order to make them of the same size as the octagonal pillars then introduced; for the lower half-capitals project nine inches on either side beyond the pillars, while in those of the choir, which are unreduced, their projection is only five. There is also reason to suppose that the other pair of octagonal pillars, those by the organ-screen, were cut out of older ones at the same time. [Illustration: EARLY ENGLISH MOULDING.] The clerestory windows are transitional, as is proved among other things by their being pointed, for purely æsthetic reasons, and not (as in the case of the north and south tower-arches) from any structural necessity. Each window has a smaller blind arch on either side of it, making a triple opening within to a single window in the wall; and the shafts of this triple opening are made to carry small attached shafts which bear the arches above. The capitals of the larger and lower shafts spread in an unusual manner, having to support a mass of walling. =The Roof= is a fine example of sixteenth century woodwork, and doubtless replaced a simpler Norman roof of wood; but the brackets which support it were added later to the Norman shafts, in order to carry a Perpendicular vault of stone, which was never carried out. It is divided into small panels, whose ornament, though rich, is rather mechanical. The nave and choir are used as the College Chapel of Christ Church. In the returned stalls by the organ-screen sit the two censors; most of the undergraduates occupy the benches of the nave (which are modern woodwork carved by Chapman after Sir G.G. Scott's designs), as far as the raised seats where the choir sits; the central benches under the tower are reserved for the freshmen; while the dean, canons, students (_i.e._, fellows), and graduates fill the stalls of the choir, the other seats of the choir being occupied by the Scholars. The public use the aisles, transepts, and chapels on Sundays, but on weekdays are free of the nave for the two special cathedral services. [Illustration: NAVE AND CHOIR, LOOKING EAST (_from a photograph by Carl Norman & Co._).]