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Lock nuts, Jam nuts, Half nuts, call them what you like.
These nuts are commonly used in conjunction with a standard nut to prevent loosening in service.
However, they are often found to be used incorrectly and thus their effectivness is dramatically decreased.
Correct use:
The thin lock nut should be used first and tightened to between 25% to 50% of the overall required torque. The thin nut is then held in place while the second thicker nut is then installed and tightened to full torque.
The science can be found here.