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 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.[1]
Tighten easily by gripping two sides
Work well in tight spaces by using needle nosed pliers
Work well in blind spots using pliers or a wrench
Can be a quick gauge to measure the position of the nut[2]
Commonly used in furniture as a blind nut, they are also used in rail channels to prevent turning of rail when pressure is applied. They are also used as a perfect foundation in metal channels for hidden fasteners.