A metric hex nut with an enlarged circular base flaring out from the bottom of the nut. The bearing surface of the flange has serrations which displace material on the mating surface when the nut is wrenched into place, forming a connection which resists loosening.
A six-sided, internally threaded fastener, designed to be used specifically with bolts of property class 10.9 and lower. J.I.S. (Japanese Industry Standard) fasteners have a smaller hex dimension than DIN metric fasteners. They fit in those tight spots where the hex of a DIN fastener is too large. J.I.S. fasteners are often used on import cars.