which the columns of the nave rest are probably pieces of the foundation of the older wall, refaced, or even left in the rough. Instances are nearly as common in which the heads of the new arches have blocked earlier windows; for, in the eleventh and twelfth centuries, when glass was rare and expensive, and the openings were usually closed by latticed shutters, the windows were set high in the wall. There is a remarkable example of the retention of old work at Seamer, near Scarborough. To this fine twelfth century aisleless church a north aisle was added in the fifteenth century. The builders, possibly wishing to avoid expense, employed the old method, which in those days of prosperity and general rebuilding had fallen into disuse. In order not to interfere with the older windows, they deliberately made their arches very low: the result is that, from the interior of the aisle, one can see that the old wall was almost entirely kept, the new columns being built up on the line of the flat pilaster buttresses, which were left unaltered above the capitals. Sometimes, the connexion between nave and aisles was made by cutting arches at intervals in the wall, without building columns. The north arcade at Billingham in Durham, and the thirteenth century arcades at Tytherington in Gloucestershire consist of arches with large masses of the earlier wall left between them. Such a method was economical, as much less dressed stone was required; and we find it employed at Copford in Essex, where good building stone was hard to get. Nevertheless, it prevented the free circulation of light from the windows of the aisles, and practically shut off the aisles from the church. [Illustration: Fig. 9. Gretton, Northants: arcade of nave showing blocked window head.]