A jam nut is a thin nut that jams against another locking it in place and preventing another nut from moving. It is most commonly used in situations where larger, more standard-sized nuts will not fit.
A jam nut comes in handy when you need to lock an item in place using a nut without the support of a second object. The jam nut is used as the "second object." A jam nut is also used to secure an item without applying any force to it.
Using a jam nut eliminates the need to introduce a second, unnecessary object simply for fastening purposes. Now, whatever you are trying to join together can be done simply by incorporating this one small nut.
Jam nuts are small, coming in at half the height of a standard nut. It measures in at 5/8 inches on average.
Contrary to common usage, the lock nut should be put on the inside of the nut you are trying to lock down.