Something so ubiquitous, long-lasting, and useful as a nut cannot help but evolve over time into several distinct variations on the original concept. Several varieties of the common nut have been developed for all manner of specialized applications. Below is a far from exhaustive list of descriptions for several of the more common varieties of nut.
"Outside end" refers to the end of the nut intended to be facing away from the work piece, that is, the end that meets the bolt last as it is threaded on.
- Common nut: The basic, everyday, ordinary nut is a thick, flat, hexagonal piece of metal with a tapped hole through the center, intended to thread onto a machine screw, bolt, or threaded rod. The nut is usually used as a second brace, along with the bolt's head, to securely hold two things together using the enormous mechanical advantage provided by a screw. The six sides provide a surface for a tool, such as wrench or nut driver, to grip so that it can be tightened against a work piece. Most nuts are six-sided, but four-, eight-, and twelve-sided versions also exist, being most useful when they match the number of sides on the bolt head. Nuts can be used in conjunction with a washer to increase their surface area and help prevent damage to the work piece.
- Non-locking nuts: These are special-purpose nuts designed to provide some additional functionality to the standard nut design.
- Acorn nut or cap nut: A nut with a domed cap over the outside end, which prevents the end of the bolt or threaded rod from protruding. This can be done to protect the threads, protect other things from the threads, or to prevent the nut from threading down too far.
- Anchor nut: A nut with one or more (typically two) flanges with holes in them, intended to be fastened to the work piece with nails, screws, or other bolts.
- Coupling nut: A long nut tapped from each side to meet in the middle, rather than all the way through in the same direction, used to connect two threaded rods end to end.
- Flanged nut: A nut with a widened base which acts as a washer, to increase surface area against the work piece.
- Free spin washered nut: Similar to the flanged nut, but the flange is free to spin with respect to the nut, as a separate washer would.
- Knurled nut: Rather than having six sides, a knurled nut is round with a pattern similar to the milled edge of a coin. This pattern provides a gripping surface so that it may be tightened by hand.
- Lug nut or wheel nut: A nut with a tapered end which fits into a flanged hole, centering the nut in the hole. These can typically be found on car wheels.
- Well nut: A flanged neoprene bushing with a nut embedded in the non-flanged side. The neoprene bushing forms a seal in the bolt hole and the nut is used to secure it.
- Wing nut: A nut with two extensions pointing up and slightly at an angle, providing grips to tighten the nut with fingers instead of with tools. Wing nuts are useful for applications that must be adjusted or removed frequently. Forged wing nuts are more expensive, but higher quality, than wing nuts stamped out of sheet metal. Stamped wing nuts have wings formed by folding the metal over, leaving a hollow area in the middle. Forged wing nuts have solid wings.
- Lock nuts: These are intended to lock into position on the thread in one of a variety of ways, to prevent failure by loosening. Most lock nuts require a significant torque to turn, more than can be generated by most mechanical vibrations and other such stresses.
- Castellated nut, or castle nut, or slotted nut: So called because the outside end is shaped in dagges, like a castle parapet, the castellated nut is designed for use with a bolt that has a hole drilled through it. After tightening, a cotterpin is to be inserted through the slots in the nut and the hole in the bolt, which prevents the nut from turning and loosening. This kind of nut can be found on a car's tie rods.
- Deformed thread nut: A nut which has been deformed at the outside end so that it is no longer perfectly circular. As the nut is threaded on, the deformed end is forced back into a circle, holding tight to the bolt and preventing it from turning.
- Jam nut: A second nut intended to be tightened against another nut to bind the threads, requiring significant torque to unbind them. This also prevents one nut from moving in the direction of the other without turning both nuts simultaneously.
- Nylon insert nut: A nut with a ring of nylon on the outside end which forms itself against the threads as it is tightened. This not only locks the nut in position, but also creates a seal against liquids and gasses. This type of nut is not intended to be reused more than a few times, as the nylon wears out. Not for use in high heat applications due to nylon's low melting point.
- Serrated face nut: A nut with ridges machined in to one end which bite into the work surface, preventing it from turning.
- Split beam nut: A nut which has slots cut in the top after it has been tapped, which separate the outside end into four or more sections which are then bent slightly inward. As the nut is threaded on, these sections are forced back out to their original position and hold tight to the bolt, preventing it from turning.
- Star washer nut: This style of nut has a free spinning, non-removable external tooth star lock washer built in. The star lock washer bites into the work surface, preventing it from turning.
- Tee nut or T nut: A nut with small spikes intended to sink into a wooden work piece, holding it in place and preventing it from turning. The bolt head will need to be turned in order to tighten this, because the spikes prevent the tee nut from turning in either direction.