Church, often large and magnificent, in which a residential bishop has his official seat. Cathedrals are usually embellished versions of early Christian basilicas; their construction, on an ever-larger scale, was a major preoccupation throughout Europe in the Middle Ages. Masonry vaulting replaced the earlier timber roofs, and the basilican plan grew more complex. Above the arches of the nave, and below the clerestory, was the triforium, an arcaded upper story that often contained vaulted tribune galleries open to the nave. The portion containing seats for the choir, usually east of the transept, was called the chancel. Between the chancel and the sanctuary (high altar) was the presbytery, a raised area occupied only by clergy. The chapter house, a popular feature of English cathedrals, was a chamber, typically octagonal, in which business was transacted. Small chapels, including the founder's chantry and the Lady Chapel (dedicated to the Virgin Mary) were often added. Many cathedrals of the Île-de-France region were remodeled to embody a chevet, or arc of radiating chapels, on the eastern wall, a feature reflected in England in Westminster Abbey and Canterbury Cathedral.