Zagane (座金) – steel washers. Washers come in a wide variety of shapes and sizes — and they’re all called zagane — but the ones that house-builders might come into contact with most often are large, round, and have a spring built onto them. There are of course many others. I will attach images of the 3 most common types house-builders use.
The first image is the one used for hagoita as well as all bolts whose fastening end comes in direct contact with wood.
This one is used for steel hiuchi (wooden ones require the one in the image above).
This is a special kind of washer that is made to remove wood as you tighten it down, and is used for dodai.