The _triforium_ is the arcaded story between the lower range of piers and arches and the clerestory. The name is supposed to be derived from _tres_ and _fores_--three doors or openings, for such is often the number of arches in each bay. Professor Willis, however, believed that the word is traced to a monkish Latin word for thoroughfare. _Clerestory_, or _clearstory_, is the upper story of the nave of the church above the aisles and pierced with windows. The windows of the clerestories of Norman work are less important than in the later styles. They become larger in the Early English period and more important in the Decorated, always lengthening as the _triforium_ diminishes. Sometimes the choir occupies two bays of the nave, but usually begins with the screen placed on the east side of the central tower. In olden days this was the rood-screen, so called because a large crucifix, or rood, stood on it. All roods were destroyed during the Reformation. At the present time the organ is frequently placed here; and there is diversity of opinion about the artistic propriety of its position. Entering the choir we see the high altar often with a reredos (French _l’arrière dos_, i.e., embroidered hangings). Along the sides of the choir are the seats, or stalls, usually of carved oak, surmounted with tracery, arches and pinnacles. Among these is the bishop’s seat, or throne. Frequently the stalls exhibit beautiful tabernacle-work and the misereres (_miséricorde_), which turn up and afford support to a person in a position between sitting and standing, are generally carved with grotesque and quaint figures and caricatures. Vestries for the use of priests and choristers are often situated near the choir. At the back of the choir (the retro-choir) was placed the chief shrine, where relics of the great saint of the cathedral were kept and to which the streams of pilgrims passed. In many churches the steps and pavements are worn away. Near the shrine was a watching-chamber, where a monk guarded the shrine and its treasures. Further east the Lady-Chapel was situated, though in a few cases it is found on the north side, e. g. Bristol and Ely.