Nuts are a fastener with an internally threaded hole. They are almost exclusively used to mate with a bolt that has the correct threading pattern. Nuts and bolts work to hold materials together by creating a temporary joint. Although the idea seems simple, nuts can be a complex product as they are offered in a variety of shapes and sizes for a wide range of purposes.
Most nuts are installed with a wrench, socket or driver, but certain types, such as knurled and wing nuts, are installed by hand. In environments subject to vibrations, lock nuts can add extra tension to an assembly to prevent accidental back off. For the most part, standard hex nuts work well with many household applications.
Square nuts were the standard in the past due to their ease of manufacturing, but as methods of making nuts has progressed the six-sided hex nut has become the standard nut. A six-sided nut allows for more entry points on the nut. This allows a wrench to work more efficiently in tight spaces. Square nuts are still used today in situations where extreme torque is required. Four sides have a much higher tolerance against stripping than their six sided sibling.
Overall, nuts can be found in a wide range of varieties which can suit needs from household applications all the way to industrial standard requirements.
Nuts in most varieties are available in both SAE and Metric.