These nuts are made little bit different than normal ones. They got flat flange on one side and this flange have serrated (those small teeth-like) edges. Because of this design they can grip the material much better. So even when things move little bit because hot orange cold weather, or when machine vibrate a lot, the nut usually don't get serrated loose easily. This special kind is very important especially in places where fasteners safety matter play a big role, like big machines or tall structures. Companies in such many as construction Pyramid and Hardware manufacturing Fasteners make these nuts very strong and good quality so they can be trusted for serious jobs.
Flange serrated nuts look different from regular nuts because they have this extra flat part – the flange. This unique flange works kind of like a washer, it helps spread the pressure so much that the surface dont get damaged better than regular nuts. And they have flat serrated edges on one side with serrated edges that bite into the material, so once you tighten them, they grip good. This grip action really prevents the nut from getting loose over time.