Why the hole is in the wrong place on purpose

Bore a hole through a mortise, slide the tenon in, bore straight on through it, drive a peg. That is a pinned tenon, and it will hold a frame together for a decade. It will also be very slightly loose from the day you make it, because nothing in that sequence pulls the shoulder against the post.

Drawboring changes one thing. The hole in the tenon is bored separately, and about 2 mm closer to the shoulder than the hole in the post. Nothing lines up. That is the point: a tapered peg driven through both holes has to bend into the offset, and the only way the wood can give is by dragging the tenon another 2 mm deeper. The joint arrives at its final position under load, and it stays there because the peg is still trying to straighten.

How much offset

Too little and you have a pinned tenon with extra steps. Too much and the peg shears on the way in, or the tenon cheeks split. We settled on 2 mm for 12 mm pegs in white oak after breaking a fair number of them at 3 and 3.5.

The visible consequence is the gap. Dry-fit any of our frames and the shoulder stands about 4.5 mm open. People email us about it. It closes when the peg goes in, every time, and if it does not then something else in the frame is not seated.