Drawbored Mortise And Tenon — What It Is, Where the Strength Comes From, and the Numbers

A drawbored mortise and tenon pulls its own shoulder tight using nothing but an offset peg hole — no clamps, and historically, often no glue at all. It's the joint behind centuries of timber framing and Windsor-style chairwork that has stayed assembled through generations without a single drop of adhesive.

The mechanism: an offset that does the clamping for you

The trick is deliberately misaligning two holes. First a hole is bored through the leg and the mortise, then the tenon is fitted in dry and the same bit is used to mark the tenon's face through that hole — except the tenon's hole is then bored slightly toward the shoulder, not exactly on the leg's mark. When a tapered peg is driven through both holes, it has to pull the tenon's hole into alignment with the leg's hole to seat fully, and that pull drags the tenon's shoulder tight against the leg's face. The peg is doing what a clamp would otherwise do, permanently, and it keeps doing it for as long as the peg stays put — which for a well-fit hardwood peg in a hardwood frame is effectively forever.

How much offset, and why more isn't better

The offset is typically 1/16" to 3/32" — for a 5/16" peg, a 1/16" offset is 20% of the peg's own diameter, which is enough draw to close a real gap without asking the peg to bend so sharply it snaps on the way through. Too little offset and the joint doesn't actually pull tight; too much and the peg has to flex more than straight-grained hardwood safely tolerates, risking a peg that shears instead of drawing.

Peg sizing and species choice

Pegs are commonly cut from riven (split, not sawn) hardwood stock 1/4" to 3/8" in diameter — riven stock follows the grain instead of cutting across it, which is what lets the peg flex through the offset without snapping. A tough, straight-grained species like white oak or hickory is the traditional choice; a peg cut from a weaker or cross-grained blank is the single most common cause of a drawbored joint failing during assembly rather than years later.

Cut sequence

1. Cut and fit the mortise and tenon exactly as for a standard through or blind joint, confirming a snug dry fit before any boring begins.
2. Assemble the joint dry and bore the peg hole through the leg and into the tenon in one pass, so the leg-side hole is a true reference.
3. Disassemble, and mark the tenon's hole location by pushing an awl through the leg's hole against the tenon face.
4. Move that mark 1/16"–3/32" toward the tenon's shoulder before boring the tenon's hole — this is the entire offset, and it's the one step that can't be skipped or approximated by eye.
5. Reassemble (with glue, if the design calls for it — drawboring works with or without) and drive a tapered, riven peg through both holes until it seats fully, watching the shoulder line close as the peg draws it home.
6. Trim the peg flush once seated.

What too much offset actually breaks

A peg driven through an offset cut too aggressively doesn't just risk snapping mid-drive — even when it survives, it can crush the tenon's own hole into an oval rather than staying round, which quietly reduces how much of the peg's surface is actually bearing against the tenon once everything settles. That's a slower, less dramatic failure than a peg that snaps outright, and it's part of why the 1/16"–3/32" range above is a genuine ceiling rather than a conservative suggestion — offset beyond it buys very little additional draw while meaningfully raising the odds of exactly this kind of quiet damage.

Why this is one of the few joints that improves with age

Wood compresses slightly at the peg hole over years of seasonal movement, which loosens most peg-and-hole fits over time — a drawbored joint is unusual in that re-driving the peg slightly further re-tightens the offset and pulls the shoulder tight again, something a plain glued joint has no equivalent remedy for once its adhesive bond starts to fail. This is part of the reason drawboring shows up so often in timber framing meant to stand for generations: the joint has a built-in maintenance path that a purely glued tenon simply doesn't.

Where this differs from a wedged tenon

A wedged through tenon spreads the tenon's own end mechanically once it's already seated; a drawbored joint does the opposite job — it pulls a joint together that hasn't fully seated yet, working through the tenon's cheeks rather than its end grain. The two techniques solve different problems and can, on heavy timber-frame joints, be used together on the same tenon.

What the careful boring buys, measured against the alternative

Laying out and boring an intentionally offset hole takes real care — it's the one step in this joint that has no shortcut and no forgiveness for guessing. In exchange, a drawbored joint clamps itself permanently, needs no clamps or glue cure time during assembly, and remains serviceable (and re-tightenable, by driving the peg further) long after a purely glued joint's adhesive has aged out.