Ken-isuka-tsugi or Bird’s Mouth Joint

The Ken-isuka-tsugi. Bird’s Mouth Joint. In Kiyosi Seike’s, The Art of Japanese Joinery, both history and catalog of Japanese carpentry & joinery, there is a spread of several black & white photographs, detailing a selection of different joints. Some very simple, others, infinitely more complicated. The photography alone is lovely, but the joints themselves are a treat to view.

I wanted to try recreating some of these joints and one that caught my attention and also looked possible, was the Ken-isuka-tsuji, or Bird’s Mouth Joint. There isn’t much readily available information online, as a cursory search will bring up primarily roofing techniques along with a few boatbuilding/spar/mast versions. All fine and good, but I was really more interested in simply making the joint to see if I could do it.

I did find a video, with none other than Jay Van Arsdale, demonstrating the process of making this joint, but it was short on details…basically just a video of the joint being cut with Mr. Van Arsdale discussing as he went. All the same, it was invaluable, just in terms of seeing the actual three dimensional process.

A good friend gave me a piece 3″x3″ poplar about 2′ long, which I used to give this puzzle a go. Hopefully, from the images below you can glean the process of how it’s cut. Essentially there are 4 cuts-that’s it. If it goes well you get this little gem of a pyramid that just falls out after the last slice of the saw blade. It’s a pretty fabulous moment.

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