“ In this more inclusive sense the early basilican churches had no chancels, which were a comparatively late development; the cancelli, e.g. of such a church as San Clemente at Rome are equivalent not to the “chancel screen” of a medieval church but to the “altar rails” that divide off the sanctuary. In churches of the type that grew to its perfection in the middle ages the chancels are clearly differentiated from the nave by structural features: by the raising of the floor level, by the presence of a “chancel arch,” and by a chancel or rood screen (see Rood) . ”

“ These screens served the purpose, in collegiate and conventual churches, of cutting off the space reserved for the services conducted for and by the members of the chapter or community. For popular services a second high altar was usually set up to the west of the screen, as formerly at Westminster Abbey. ”