Nominal diameter is also called nominal size. The actual size of any object can not be absolute, such as a round rod length 100, diameter 30 are nominal size, and the actual size may be 100.1, 99.99, 99.82, 30 May be 29.999, 30.125 and so on.
This is especially true when it comes to threads, the nominal size of the stud and nut is G1\/2, and the actual outside diameter of the stud must be less than G1\/2, and the large diameter of the nut must be greater than G1\/2 in order to effectively match.
The concept of the middle diameter is the conical middle line of the nut and the stud, which is not redundant.
Specifically to the G1\/2 pipe thread you said, its meaning is: inch pipe thread, the nominal diameter of the stud is 25.4X1\/2 that is 12.7, the actual size is 12.6. 12 teeth per inch means the distance between each tooth is 25.4\/12.
The large diameter of the nut 12.8 bottom hole diameter is 12.7- tooth Angle coefficient X(25.4\/12), the specific calculation description is more complicated, please refer to the relevant table.