This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
Double-hung sashes are divided into two parts, one called the "upper sash" and the other the "lower sash," which are so arranged as to slide by each other. They meet at the center of the window opening, and at this point, at the top of the lower sash and at the bottom of the upper sash, is a rail known as the "meeting rail." In Fig. 316 is shown a section through the meeting rails of a window. The section has been taken vertically and shows the meeting rails at a large scale. A is the top rail of the lower sash and slides up. B is the bottom rail of the upper sash and slides down, the two coming together in the inclined line marked C. Each rail is cut so that when they come together they will meet in this line. The thickness of the rails is determined by the fact that the distance marked D is 1 inch, making the entire thickness of the rail B 1 3/8 inches and the thickness of the rail A 1 3/4 inches. The rail A is carried down below the bottom of the rail B so as to allow the glass to be puttied in as shown at E. In Fig. 317 is shown another method of fitting the glass into the top rail of the lower sash. Here A is the top rail of the lower sash and at E is shown the method of fitting the glass. As will be seen, the rail A is ploughed to a depth of about 1/4 inch and the glass inserted in the opening. This method allows the rail of the lower sash as well as the rail of the upper sash to be only 1 3/8 inches thick. Fig. 317 also shows another method of constructing the meeting rails as shown at C. Here, instead of meeting in a straight line as in Fig. 316, there is a slight rabbet made in each rail so as to give a small extent of horizontal surface on each. The advantage of this method is that it prevents the sashes from slipping too far past each other, as they may do if cut as shown in Fig. 316, especially after they have become a little worn.
At the corners, where the horizontal rails meet the vertical stiles, they are fastened together with a mortise-and-tenon joint, the mortise being in all cases cut in the stiles and the tenon made on the ends of the rails. This is shown in Fig. 318 where at A is the joint between the top rail and the stile, and at B the joint between the meeting rail and the stile. D is the top rail and E is the stile, while at H is the tenon cut in the end of D, fitting into a mortise in E. F is the meeting rail tenoned into the stile. It is a common practice to continue the stile some distance below the meeting rail and to cut a molding in the end of it as shown at C. This makes the stile much stronger at this otherwise weak point. The joint between the bottom rail and the stile is made in a manner similar to that shown at A.
Fig. 317. Another Form of Meeting Rail.
Fig. 31S. Joint Between Stile and Rails.