Hagoita (羽子板) – a steel plate welded to a threaded bolt used to pull and fasten two perpendicular structural members together. From the images below you can see how this works. Here in Japan, wooden beams are often notched and fit together, but need additional fastening in the form of Hagoita.
Hagoita typically come in several pieces — the main plate and bolt (as shown in the first image), a second bolt that is passed through in the perpendicular direction, and the accompanying nuts and spring washers (zagane).
When fastening hagoita, always tighten the bolt perpendicular to the main plate bolt first. Tightening the plate bolt first will pull the still loose perpendicular bolt out of its right angle position, resulting in an inferior connection.