Metric threads include ordinary threads (profile angle 60°), trapezoidal threads (profile angle 30°), zigzag threads (profile angle 33°), square threads, etc.

The specification of coarse thread ordinary thread is expressed by the letter "M" and "nominal diameter". The specification of fine thread ordinary thread is expressed by the letter "M" and "nominal diameter When the thread is left-hand, add the letters "LH" after the specification.

S.W. Standard Whitworth coarse thread series, general purpose cylindrical thread British Standard.

S.F. Standard Whitworth fine thread series, general purpose cylindrical thread.

The tooth type symbol of ordinary thread is represented by "M". The thread code of a coarse-thread ordinary thread is represented by the tooth type symbol M and the nominal diameter (major diameter) (the pitch is not marked), such as M16; the fine-thread ordinary thread is represented by the tooth type symbol M and "nominal diameter X pitch", such as M16* 1.5; right-hand thread is a commonly used thread, and the direction of rotation is not marked; left-hand thread requires "LH" after the size specification, such as M16*1LH.

The thread tolerance zone code includes the pitch diameter tolerance zone code and the top diameter tolerance zone code. It consists of a tolerance grade number indicating its size and a letter indicating the basic deviation of its position (uppercase letters for internal threads, lowercase letters for external threads), such as 6H, 6g. If the codes of the pitch diameter tolerance zone and the top diameter tolerance zone are different, note the codes respectively, with the code of the mid diameter tolerance zone first and the code of the top diameter tolerance zone last, such as M10-5g6g; if the pitch diameter and top diameter tolerance zones are the same , then only enter one code number, such as M10*1- 6H. When the internal and external threads are combined into a thread pair, the matching tolerance zone codes are separated by a slash. The left side represents the internal thread tolerance zone code, and the right side represents the external thread tolerance zone code, such as M10-6H/6g.