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A nut is a type of hardware fastener with a threaded hole. Nuts are almost always used opposite a mating bolt to fasten a stack of parts together. The two partners are kept together by a combination of their threads' friction, a slight stretch of the bolt, and compression of the parts. In applications where vibration or rotation may work a nut loose, adhesives, safety pins, and other tricks are used to prevent fastener rotation. The most common shape is hexagonal, for similar reasons as the bolt head - 6 sides give a good granularity of angles for a tool to approach from (good in tight spots), but more (and smaller) corners would be vulnerable to stripping/rounding. Other specialized shapes exist for certain needs, such as wing nuts for finger adjustment and captive nuts for inaccessible areas.

Nuts are graded with strength ratings compatible with their respective bolts; for example, an ISO property class 10 nut will be able to support the bolt proof strength load of an ISO property class 10.9 bolt without stripping. Likewise, an SAE class 5 nut can support the proof load of an SAE class 5 bolt, and so on. The proof strength of the most common property classes is listed at bolted joint.

L to R: Wing, hex, hex flange, and flanged weld (for stud welding) nuts.

L to R: Slotted, square, T, cap (or acorn), nylon locking, and castellated nuts.

• Aerotight® "stiff nut"
• castellated nut
• flange nut (collar nut)
• hex nut
• insert nut
• jam nut
• KEPS® nut
• knurled nut (thumb nut)
• lock or locking nut (Nyloc®)
• lug nut
• oval lock nut (Cleveloc®)
• PEM® nut (for metal)
• polymer insert nut (Nyloc®)
• Rivnut®
• slotted nut
• speed nut (stamped from sheet metal)
• square nut
• staked/welded nut (for plastic)
• T-groove nut
• T-nut
• turnbuckle
• weld nut
• wing nut

• Tap
• Screw
• Bolted joint
• Washer
• Cage nut
• Pipe cap