Mysterious Pillar Joint of Osaka Castle Otemon Gate: The Mystery of Mortise and Tenon

The mortise and tenon joint structure used for the main pillars of Osaka Castle's Otemon Gate has long been one of the mysteries in Japanese architectural history. This model is a physical replica of this architectural enigma.

The gate pillars of ancient buildings would rot due to erosion from rainwater. Therefore, craftsmen would regularly replace the lower sections of the pillars with new materials. The Otemon Gate of Osaka Castle also underwent pillar replacement, connected using mortise and tenon joints. The lower pillars on the east and west sides (see photo) feature a convex “ari-tsugi” (dovetail joint), while those on the north and south sides (see photo) have a mountain-shaped “sogi-tsugi” (scarf joint). From the outside, the shape of these joints appeared impossible to insert from any direction, remaining a mystery. It wasn't until 1983, when X-ray imaging revealed the internal structure, that the mystery of the joints was finally solved.

As shown in the figure, it is inserted by sliding diagonally upwards from the bottom. This mortise and tenon joint is very strong and practical, and its joining method is not visible from the outside. Perhaps there's also a playful spirit in the work of these professionals.