When tightening a nut, their square neck prevents them from spinning. A rounded, smooth head gives it a polished appearance. 

steel bolts and are suitable for high-stress applications such motors, pumps, valves, and suspension systems for automobiles. These bolts have a square neck that prevents them from rotating when a nut is tightened. 

When tightening a nut, a knurled neck holds onto the inside of the hole to prevent them from twisting.