A Joint With No Nails That Has Held Buildings Up for Seven Thousand Years
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Cutting a projecting tongue on one piece of timber and a matching hole in another produces a joint that resists twisting, stays tight as wood moves and needs no fastener at all.
What the joint is
One piece is cut with a rectangular projection on its end. The other has a matching rectangular hole cut into it. The projection enters the hole and the two are drawn together, with the shoulders around the projection bearing flat against the face of the other piece. The load is carried by wood bearing on wood across a large area rather than by any fastener. Fitted well, the joint is tight enough to hold by friction alone, and it is usually secured with a wooden peg driven through both pieces.
Why it works so well
Several properties fall out of the geometry:
- •The shoulders resist the joint twisting out of square
- •Load is spread across a large bearing area rather than a point
- •A peg through both pieces resists pulling apart
- •Boring the peg hole slightly offset draws the joint tight as it is driven
- •Wood shrinks across the grain, and both parts shrink together
- •No metal means no rust and no splitting from corroding fasteners
How old it is
The joint is among the oldest surviving woodworking techniques and the dates keep moving earlier. Examples have been recovered from a waterlogged Neolithic well in Germany dated to around seven thousand years ago, cut with stone tools. Egyptian furniture uses it throughout. Chinese and Japanese timber framing developed it into an elaborate family of variants capable of assembling an entire building with no metal at all, several of which have stood through earthquakes for over a thousand years. European timber framing relies on it completely.
How it is cut
The order of operations is fixed by what can be corrected, which is a general principle in hand work. The hole is cut first, by chopping down with a chisel across the grain and levering out the waste, because a hole is difficult to enlarge accurately and easy to cut slightly small. The projection is then cut to fit that particular hole rather than to a drawing, sawn slightly oversized and pared back with a chisel until it enters with hand pressure. Every joint in a traditional frame is therefore unique to its position, which is why the parts are numbered before assembly.
The variants and what they solve
The basic form is modified for particular jobs and the variations are worth recognising. A through version passes the projection right across and out the other side, where it can be wedged, which is stronger and visible. A haunched version leaves a short additional step to resist twisting in a frame. A blind version stops short and is invisible. An angled version resists a pulling load by hooking rather than by relying on the peg. And a loose version uses a separate piece fitted into holes in both members, which is easier to cut with machinery and is what most modern joinery uses.
The takeaway
A projection on one timber entering a matching hole in another carries load across a large area of wood, with shoulders resisting twist and a peg resisting pull, and boring that peg hole slightly offset draws the joint tight. Both parts shrink together, so it stays tight as wood moves. Examples cut with stone tools survive from seven thousand years ago.