A square nut is a four-sided nut. Compared to standard hex nuts, square nuts have a greater surface in contact with the part being fastened, and therefore provide greater resistance to loosening (though also greater resistance to tightening). They are also much less likely to become rounded-off after repeated loosening/tightening cycles. Square nuts are typically mated with square-headed bolts. Square nuts are used along with flat washers in order to avoid damage from its sharp edges and increase the strength of the fastener. Square nuts can have standard, fine or coarse threading with platings of zinc yellow, plain, zinc clear, tin and cadmium, among others. Most can meet either the ASTM A194, ASTM A563, or ASTM F594 standard.
Square nuts are four-sided nuts as opposed to the more common hex (six-sided) nuts. The surface area on the flats is larger than hex nuts, which reduces wrench slippage. However, you cannot use hex drivers such as sockets and nut drivers. Machine screw square nuts are available in smaller sizes and also smaller in width and thickness than regular square nuts. Machine screw square nuts have flat tops and bottoms—the edges and corners are not chamfered (beveled). Regular square nuts have a flat bottom bearing surface and a crowned washer-faced top. Regular square nuts are often used with square bolts.