Fig. 100 shows the usual method of constructing the mullions and transoms of double hung frames in wooden buildings, the details given being adapted to the frame shown in Fig. 96. The outside architrave is continued under the sill of the transom, and the moulded part, M, may be cut into dentiles if desired. The transom sash is shown over the inner of the sliding sash, which is the best position where the sash is to open. The best way of opening transom sashes is to hinge them at the bottom, as shown at H, as the joint can be made quite tight in this way, while if they are hinged at the top, it is difficult to make the lower joint weather proof. The groove in the bottom rail of the transom sash, and the rebate in the sill, as shown at R, are desirable features. Mullions are commonly made the same as a double jamb, with a partition in the middle to separate the weights. For 1 ¾-inch sash, hung with round weights, the least width of a mullion between outer faces of pulley stiles should be 7 inches. The partial section at S is taken through the pulley stile above the transom.
Where stone or brick mullions are used, a separate frame is required each side of the mullion, but the stone transom usually projects only through the outside casing, the jambs or boxes extending the full height of the window. The upper section in Fig. 101 is taken through the side of the frame above the transom. The transom sash, if stationary, may be put next the outside casing, but if it is to be hung at the bottom, the best place for it is at the inside of the frame, as shown in the figure.
In Fig. 101, a panel set flush with the box casing veneer is shown for the finish of the mullion, but if preferred, the mullion boxes may be finished the same as the jambs, with sub-jambs extending to the face of the plaster. The method shown, however, obstructs the light less and gives the appearance of one window, while the boxed mullion gives the appearance of two or more windows set side by side. When the stone mullion is not as thick as the wall, steel beams should be placed over the window to carry the weight of the wall above. The " reveal " of a window has no effect upon the construction of the frame, but the greater the reveal the less will be the depth of the inside finish.