The three wire method is recommended for universal use in the direct measurement of thread gages and components. It has been found to be the most accepatble and universally recognized method of measuring pitch diameter.
The pitch diameter is the diameter where the thread thickness is equal to the space between the threads. If the flats at the top and the bottom of the thread are the same, the pict diameter will coincide with the middle of the sloping side of the thread.
The pitch diameter is represented by the letter E
E=D - depth of thread = D - h