A Flange nut is a nut that has a wide flange at one end that acts as an integrated, non-spinning washer. This serves to distribute the pressure of the nut over the part being secured, reducing the chance of damage to the part and making it less likely to loosen as a result of an uneven fastening surface. The nuts are mostly hexagonal in shape and often coated with zinc.
The flange may be serrated to provide a locking action. The serrations are angled such that they keep the nut from rotating in direction that would loosen the nut. They help in preventing the vibration of the nut from moving the fastener, thus maintaining the holding power of nut.