First built in the 1500s, Osaka Castle has been destroyed, rebuilt and repaired over the years. In the early 1900s one of the wooden columns supporting the castle's main gate, known as the Otemon Gate, rotted out at the bottom. Craftsmen patched this in 1923 with this peculiar splice:

I know the photo quality is poor. Here's a better photo of an identical splice joint executed by craftsman Chris Hall:

Image and work: Chris Hall

Following World War II, this splicing technique had been lost. Japanese craftsmen in the latter part of the 20th century had no idea how this splice was installed.

In 1983, the joint was X-rayed. The joint was reverse-engineered, and Japanese craftsmen (and eventually Western craftsmen like Chris Hall*) learned to make it again.

Here's how the mysterious joint goes together. I've cued this up to the right spot:

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The joint is referred to both by its technical name, basara-tsugi, and colloquially as the "Otemon splice."

It's one thing to see how it goes together; it's another to make the joint on-site. Imagine trying to chisel that thing out, upside-down, on the rotted column while it's in place. I imagine installation on-site is similarly fiendish; if you cannot jack the entire structure up, you'd need to remove foundation stones (in Japanese timber framing, the posts often rest on stacked stone pilings dug into the ground) to have the space to wedge the replacement part in.

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*Chris Hall was a highly skilled craftsman who shared his knowledge on his website, The Carpentry Way, until his passing in 2020. Hall's widow maintains the website in Chris' memory. If you're interested in woodworking, I highly recommend checking it out.