In thick walls and rich work there is often another order of through-arch within the tracery order, or rather the outer order re-appears within. The rere-arch is occasionally cusped, as in a window at Broughton, Oxfordshire (Fig. 165), and the intervening space is sometimes groined, as in some windows at Salisbury and Christchurch (Fig. 166), or richly panelled, as in some at Westminster. In some instances the place of the rere-arch is occupied by distinct tracery, like a second window in advance of the real one. This consists in most instances of perfect bar tracery, while the window itself is of plate tracery; as may be seen in some of the windows at Stone Church, Kent (Fig. 167), and as once existed on a much larger scale in the chapter-house at Tintern. I may here mention that the tracery of a window is always viewed as an _arch-order_; and, though the corresponding order in the jamb is in the solid with the jamb up to the springing, the tracery, like other arch-orders, is severed by a continuous joint from the order above it. [Illustration: Fig. 167.--Stone Church, Kent.] The most normal type of the through-arch window is that in which the glass is placed in the middle of the thickness of the wall, and the interior of the window is a mere repetition of its exterior. This is not, however, by any means necessary or constant; for the glass is often either less or more recessed, and the inner mouldings, etc., are not always similar to the external ones, so that the existence or non-existence of a separate internal arch is the more clear distinction. Some, however, of an intermediate character, are to be found in which an inner arch, separate in design, is nevertheless concentric with the outer arch. In others the separate existence of the inner arch arises from the existence of a triforium passage, which in clerestory windows leads to some changes of design from the normal type. In others the rere-arch is not only concentric, but is so close upon the outer arch as to be almost one with it. The two classes are, however, for the most part easily distinguished. [Illustration: Fig. 168.--Chancel, Brecon Priory.] One of the earliest instances which I recollect of the rere-arch is in the eastern part of Tynemouth Priory.[63] This is in the transitional style, and the strongly-marked separation of the inner from the outer arch is largely owing to the vast thickness of the walls. The glass plane is perhaps four times as far from the inside as from the outside.