The main purpose of the nut is to allow the maximum amount of torque and grip for a given size to tighten against a part. Nuts are graded with compatible bolts or screws.

Following are the most commonly used nuts:

Hex Nut.
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Lock Nut or Jam Nut.
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Cap Nut.
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Castle Nut.
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Coupling Nut.
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Flange Serrated Nut.
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K. Nut.
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Square Nut T.
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T Nut.

Shear Nut.
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Wing Nut.
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Knurled Thumb Nut.
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Nylon Insert Nut.
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T Nut.
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The most commonly used nut is the six-faced Hexagonal nut. A jam nut is the second nut that is used to keep the first nut locked in place. A cap nut is used to protect the threads of the screw it covers. A castellated nut, also called a castle or slotted nut is used to insert a cotter pin from preventing the nut to rotate. A coupling nut is used for joining two male threads. A flange nut has a wide flange at one end which acts as an integrated washer. The serrated flange nut grips the part being tightened. A k-nut has an attached free-spinning lock washer. Square nuts provide a greater surface contact area and are typically mated with square head bolts. Wingnuts are threaded nuts with wings on each side of the body allowing for easy manual turning operation. Shear nuts are cone-shaped nuts with a hexagonal top. Once maximum torque is reached the hex head snaps off leaving behind a protective cone nut that cannot be easily removed. A knurled thumb nut has a rounded but knurled outside surface which facilitates tightening by hand. A nylon insert lock nut is internally threaded with a nylon insert to prevent loosening from vibration. A t-nut is used to pierce into the material and be flush with the surface.

Swage Nut.
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Rivet Nut.
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Weld Nut.
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Cage Nut.
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A Swage nut or clinch nut is used to clinch to a sheet to provide threading. Rivet nut has serrations and is riveted t the sheet, weld nuts are welded to sheets. Cage nuts come with housing which will be clipped onto a square slot provided in a sheet.