Square nut

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.

[1] Benefits claimed for square nuts include that they 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.