Building Doors and Windows
The names of the parts of a flat that we use in the theatre were adapted from the names of the parts used to construct a panel door. It is not possible to build a door that is one solid piece of wood. Of course trees don’t grow that big, but even if they did, the resulting structure would be very unstable. You have seen how an ordinary 1x12 tends to cup out of shape because of the width of the board and the way that the growth rings dry out. Imagine how much cupping would occur if the board were 36 inches wide. Also, you recall how wood tends to swell across the grain when the weather is humid. If the door were really one solid piece of wood with the grain running vertically, it would tend to shrink or swell a tremendous amount depending on the humidity. So the proportionally slender rails and stiles are used to form the outer structure of a door, and the much wider interior panels are loosely joined inside a mortise, so that they can change in size without affecting the rest of the door.