Square nuts are four-sided, internally threaded fasteners, mated with bolts, studs, screws, and other externally threaded products. The addition of the square nut ensures that the bolt, etc., stays in place. Square nuts are most commonly mated with square bolts.
Square nuts offer a greater surface area than do hex nuts, and therefore provide greater adherence. A disadvantage is that they require more effort to tighten.
A small version of the square nut, used with machine screws, is called the Square Machine Screw Nut.