Rule 25 of 31 · Chapter V — Joinery and Glue-Ups
Reinforce joints under real load
Why this rule exists
A joint that looks fine can fail where forces concentrate. Chair rails, table aprons, and door frames take racking loads that pull joints apart over years of use, and a plain glued butt joint won't hold there. Adding a mechanical component, a tenon, dowel, domino, or well-placed screw, gives the joint long-grain glue surface and mechanical resistance. Knowing where to reinforce, and where not to bother, is the quiet difference between furniture that lasts and furniture that wobbles.
In practice
Put your strongest joinery where the loads concentrate: mortise-and-tenon at chair and table rails, at door frames, at anything that gets leaned on or racked repeatedly. Favor joints with long-grain-to-long-grain glue contact, since end-grain glue joints are inherently weak and starve for adhesion. Where you genuinely can't cut real joinery, reinforce with dowels, biscuits, dominoes, or screws placed to resist the actual direction of the load, not just added somewhere for comfort or out of vague worry.
When it doesn't apply
Not everything needs heavy joinery. Low-stress parts, decorative trim, drawer bottoms, and back panels are fine with glue and brads or simple rabbets. Over-building wastes effort and can even weaken thin parts by cutting them away. Reinforce in proportion to the real forces the joint will see, not out of blanket caution.