What Is a Butterfly Joint? The Woodworking Technique Behind the Bow Tie Key
A small piece of wood, shaped like a bow tie, set into the face of a board. It is a distinctive shape you will see again and again in woodworking, and it has a name. In English it is a butterfly joint, or a butterfly key. In Japan it is called chigiri.
What Is a Butterfly Joint?
A butterfly joint is a woodworking technique: you inlay a bow-tie-shaped piece of wood, called a butterfly key, into the surface of a board. It is used mainly to reinforce a crack or a glue seam.
Why the Bow Tie Shape?
The key is set so that it bridges the crack or seam. Because the waist is narrow and both ends flare out, the key cannot pull straight out of its recess. That is what holds the two sides together and keeps the crack from opening any further.
What Other Shapes Can a Butterfly Key Take?
The bow tie is the shape most people picture, but a key can be cut to almost any outline. Shape it to suit the design of your piece and the spot you are setting it into.
How Is a Butterfly Joint Used?
Wood moves. As a solid slab dries out, or as the humidity around it shifts, it can crack. When you want to use a slab with cracks for a tabletop or similar surface, a butterfly key is set across the split to stop it from traveling any further. Keys are also set across a glue seam purely for the look, turning that distinctive shape into part of the design.
Can You Cut a Butterfly Joint with Hand Tools Alone?
Yes. You do not need a router or any other power tool. Power tools are quicker, no question — but shaping the key a little at a time with a saw and a chisel is the real pleasure of doing it by hand.
How a Butterfly Joint Is Made
First you cut the key itself, then chop a recess in that exact shape into the board. Glue the key in place, and finish it flush with the surface.
For the full step-by-step, see How to Make a Tea Mat with a Butterfly Key.
Hand Tools for Cutting a Butterfly Joint
Here are the hand tools you will use, from cutting the key to setting it into the board and finishing it flush.
Marking Tools
Use a carpenter's square to scribe the key's dimensions and angles accurately. For the angled sides of the waist, a protractor lets you set the angle exactly.
Saws
Ryoba Saw (Double Edge Saw)
Used to cut the key to shape. The corners where the angled faces meet have to be cut cleanly, so a fine-toothed saw is the right choice.
Ryoba Saws with One-Touch Blade Replacement
Flush Cut Saw
Used once the key is seated, to trim it down level with the surface of the board.
Kakuri Mini Craft Saw -Flush Cut
Chisels
Used to chop out the recess that the key sits in.
Oire Nomi Chisels -White Paper Steel #2
Planes
Used to bring the key and the surrounding surface down flat together. On a wide surface like a tabletop, a plane really earns its place.
Tips for a Clean Butterfly Joint
A clean result comes from working accurately at every stage — shaping the key, chopping the recess, seating it, and finishing. Here is what to watch for at each step.
Scribe Your Dimensions and Angles Accurately
How precisely you scribe decides how the finished joint looks. Use a carpenter's square and a protractor to lay out exact dimensions and angles. When you make the key, start with stock a little larger than the finished size. That extra margin makes it easier to cut crisp corners. Mark a center line or center point to work from and scribe both halves symmetrically; that is what gives you a balanced, good-looking shape.
Cut Accurately with the Saw
A butterfly key has angled faces and corners that have to be cut precisely. Clamp the workpiece square, and your eye, the saw, and your marking line all fall into one straight line, so you can think about nothing but sawing straight. At the corners, cut carefully and stop short of the scribed line.
Chop the Recess to Match the Key
Transfer the outline of the key onto the face of the board exactly. Hold the key still while you do it; removable adhesive or a clamp will keep it from shifting. A tool that can score along the outline, such as a kiridashi knife, gives your chisel a guide to work to and makes the job much easier.
Chop the recess a little at a time, following that line, and aim for a snug fit. Too loose and you will see a gap, so test-fit the key as you go and take the recess out in small steps. Keep the bottom flat as well: an uneven floor keeps the key from seating straight and leaves it rocking.
Seat the Key in the Recess
Shaving the four bottom corners of the key back just a hair makes it much easier to start. Tap it home evenly, alternating from side to side, so it goes in level rather than tipping.
Finish It Flush
Lay a flush-cut saw flat against the surface and slide it along to cut off the part that sticks out. A pass with a plane then settles the key into the surrounding surface. The grain in the key often runs in a different direction from the grain in the board, so read the grain as you plane. If tear-out is a problem, reach for a double-blade plane with a chip breaker.
Wrapping Up
We have walked through the hand tools a butterfly joint calls for, and the habits that make one come out clean: scribe your dimensions and angles accurately, then work patiently with a saw and a chisel.
Why not try a butterfly key in your next project?
At Kakuri, we made a chashiki (Japanese tea mat) that pairs a cherry board with a karin butterfly key. You can follow the whole build in this article.
How to Make a Tea Mat with a Butterfly Key
Frequently Asked Questions About Butterfly Joints
What is a butterfly joint?
A butterfly joint is a woodworking technique in which a bow-tie-shaped piece of wood, called a butterfly key, is inlaid into a board. It is used mainly to reinforce a crack or a glue seam.
What are the typical dimensions of a butterfly joint?
There is no set size. You size the key to the area you are reinforcing and to the board it is going into.
How thick should a butterfly key be?
Size it to the board and to how deep you are setting it. Make the key slightly thicker than the recess while you are working, then finish it flush with the surface at the end.
Can you make a butterfly joint without a router?
Yes. You can cut one with hand tools: a saw and a chisel.
