A nut is a fastener used in conjunction with a bolt or screw, providing connection and locking functions through its internal threads. It is widely used in machinery, construction, automotive, and other fields. Depending on the structure, function, and application, nuts can be classified into various types to meet anti-loosening, pre-tensioning, sealing, or special installation requirements. Common classification criteria include shape, material, thread specifications, and special processes. When selecting a nut, factors such as load strength, environmental suitability, and installation efficiency should be considered.