A mortise is a slot or trough, usually in the edge of a piece of wood. If it’s toward the end of a piece of wood it can be open on the end (open mortise) or closed (closed mortise). It can go all the way through (through mortise) the piece of wood or only part way through (blind mortise). It can have round (round end) ends or square ends (square mortise). The most difficult type of mortise to make is a true square, closed, through mortise. And that’s exactly the type of mortise used on the top and bottom rails on traditional windows.
A tenon is a tongue of sorts on the end of a board. It’s made by removing stock from either side of the tongue leaving shoulders. There are a number of ways to make a standard tenon with square shoulders. However, in the case of windows or any other project that has a profile, one of the shoulders must be coped for the parts to fit properly. This is much more difficult and it requires specialized equipment.
Unfortunately, just making a mortise in one board and a tenon on another doesn’t mean the job is done. If it’s a “closed, through mortise” the tenon won’ fit properly without some additional mill work called “Haunching”.
Haunching means cutting off a portion of the tenon in order for it to fit properly into the mortise.
Not only must the waste portion be removed but a portion of the rail has to be relieved to accommodate the profile. This cannot be accomplished by merely sawing off the waste but again requires some specialized equipment.
Each of the operations mentioned on this page must be performed with the utmost care.
If the tenons are not properly positioned, the window will be too wide or too narrow. Likewise, the position of the mortises on the stiles partially determine the height and glass size of the windows.