Jack miters are a hybrid joint, combining a small miter with a much longer butt joint. They're commonly used on quirk-and-bead casing. As Jim Chestnut, a seasoned New England carpenter, explained to me, before modern miter saws were available, cutting a long miter was a tough task. And miters in wide moldings tend to open, sometimes a lot, when moisture levels change. During the winter, when the weather is cold and dry, wood shrinks and the short point of a miter opens. When the humidity and moisture content rises in the summertime, the long point often pulls apart. Butt joints are a lot less susceptible to problems from wood movement, and if they do move, the joint doesn't open across the face like a miter, it just cracks the paint a little along a line of no visual importance.
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