will be that the piers of one arcade will be out of line with those of the arcade opposite. That discrepancies of this kind were sometimes the result of intention cannot be denied; but there is generally some practical reason to be found for the intention, and the discrepancies themselves were a _pis aller_ which the builders would have avoided, if they could. That deliberate irregularity with which medieval masons are sometimes credited is a fancy, which careful consideration of the circumstances will dispel. Sec. 46. Hitherto we have spoken of the aisled nave as though both aisles were planned at one and the same time. This, however, was by no means always the case. At Gretton in Northamptonshire, the north aisle was built soon after the beginning of the twelfth century: the south aisle followed twenty or thirty years later. The north arcade at Northallerton is of massive twelfth century work, with rounded arches: the south arcade was added in the thirteenth century, and has slender columns with pointed arches. In such cases, the north aisle may have been built first, to avoid interference with the burial ground south of the church. Very often only one aisle was added. The little church of Whitwell, Rutland, has a south aisle, added in the fourteenth century, with a chapel at its east end. No north aisle was built: but a drain in the north wall of the nave shows that there was a third altar against the north side of the rood screen. Usually, when one aisle was built long after another, the spacing of the new arcade was made to correspond with that of the old. If the old arcade had heavy twelfth century columns, the new one, with its lighter columns, would have broader arches. But it sometimes happens that the old spacing was disregarded, for very good reasons. The north arcade of Middleton Tyas church, in north Yorkshire, consists of six bays: the columns are heavy, the arches low and round headed, and very narrow. The interior of the church must have been very dark; and the builders of the south aisle, in the fourteenth century, aimed at throwing more light upon it. They therefore planned their new arcade, with broad pointed arches springing from octagonal columns, in four instead of six bays, and so, from broad windows in the aisle, introduced the necessary light. Something of the same kind happened at Theddingworth in Leicestershire: the effect is, of course, one-sided, but in both cases the light admitted enhances the merits of the earlier arcade, which, until then, had to be taken on trust.