Structural joints are formed by cutting complementary projections (tenons) and cavities (mortises) so that wood members hold each other through their own shape, with wooden pegs or wedges locking them rather than nails or glue. The resistance to pulling, racking, and shear comes from the fit of the cut surfaces themselves.
Joint selection is governed by the direction of force a member must resist: tension-resisting joints use interlocking hooks or dovetail shapes, compression joints rely on flat bearing surfaces, and joints at corners or splices combine shapes to resist multiple directions at once. Choosing the wrong joint for the load fails even if the cutting is precise.
The strength and tightness of a finished joint are set at the marking-out stage, because every saw and chisel cut only removes material to lines already scribed; errors in measurement or in squaring the reference lines cannot be corrected later without weakening the fit. Precision in the invisible layout work, not force in assembly, produces a joint that holds.