Most often used on machine screws and threaded rod, these general purpose nuts are for low-strength assemblies. Hex Nuts have flat tops with chamfered corners; bottoms may be flat or chamfered. Hex drive systems are driven with a wrench.
PTFE (polytetrafluoroethylene) has similar properties to Teflon, including an extremely low coefficient of friction (it doesn't stick to other materials and it resists surface wear). PTFE also exhibits a useful service life from below -100 degrees Fahrenheit, to temperatures of over 500 degrees Fahrenheit. Its resistance to solvents is also excellent throughout a wide range of temperatures. PTFE has a high dielectric strength (600 volts/millimeter) which makes it a good insulator. PTFE is also very non-hydroscopic, or water resistant, absorbing less than 0.01% of its weight if exposed to water for a 24 hour period. PTFE has below average tensile strength, at 3000 psi (pounds per square inch).
A threaded fastener's size name includes information about the major external diameter, followed by the threads per inch, which indicates if it is coarse or fine.