The westernmost bay of the aisle has been divided into two storeys, the upper of which now contains part of the mechanism of the organ, but is thought to have been once a chantry chapel. This curious chamber is reached through a pointed doorway at the top of the Library staircase in the south transept. Its roof is of course formed by the aisle-vault, which originally extended, doubtless, as far westwards in this aisle as in the other. The space, however, has been shortened by the great thickness of a Perpendicular cross-arch, which, though its southern respond obtrudes into the aisle below, is itself only visible from this chamber. When, therefore, the vaulting here was rebuilt, it had to be adapted to the shortened space, and the groin-ribs, which are very much of Archbishop Roger's pattern, spring from Perpendicular corbels carved into faces. The wall which separates this bay of the aisle from the choir was said above, quite truly, to be Perpendicular, but on this its southern face the masonry is apparently Archbishop Roger's. It is of gritstone, and behind the organ-bellows there remains a corbel like those of the cross-arch that props the vaulting in the corresponding bay of the north aisle. The presumption therefore is that the original vaulting was similarly propped here, and that the wall on which this corbel remains was built to block or strengthen the first choir-arch, and has survived the arch itself. To the west of the door a small square window looks into the Mallory Chapel.