"Nut or Hex Nuts are Hexagonal shaped [6 sides] nut with internal screwthread.A nut is a type of fastener with a threaded hole.Nuts are almost always used opposite a mating bolt to fasten a stack of parts together.
The two partners are kept together by a combination of their threads friction, a slight stretch of the bolt, and compression of the parts. In applications where vibration or rotation may work a nut loose, various locking mechanisms may be employed: Adhesives, safety pins or lockwire, nylon inserts, or slightly oval-shaped threads.
The most common shape is hexagonal [6 sides] give a good granularity of angles for a tool to approach and also smaller corners would be vulnerable to being rounded off. It takes only 1/6th of a rotation to obtain the next side of the hexagon and grip is optimal."