Most thread gages are of the male variety. They consist of an octagonal shaft that lists the size of the gage, either metric or standard units, and one or two threaded heads on the end(s) of the shaft that help you test your equipment. However, there are also female thread gages. These are more unusual, and they may be used in conjunction with the male thread gages, or used separately to test threaded objects. They look like flattened metal discs, about the size of a nickel to the size of a quarter, with a center hole and a cut away from the outside edge to allow room to pass over the shaft of the threaded object being tested. Here is more about the differences and sizes of both male and female gages.
The Pitch Diameter
Thread gages are measured by diameter, and by pitch diameter. The diameter is the distance across the opening of a female gage, or across the head of a male gage. The pitch diameter, however, is the distance between the threads as it spirals up/down in the center of the gage or around the head of the gage. The pitch diameter often indicates how narrow or how wide apart the threads are.
For example, you can expect that a .0514 pitch diameter to be very, very narrow. This means that the valleys of the raised thread are extremely close together and hair-thin. Likewise, you can expect the pitch diameter of .4720 to have very wide, spread apart thread valleys. This is important when you are trying to choose a thread gage to fix a machine that has lost several screws or bolts and you have to get exactly the right size and pitch diameter for the replacements.
The Actual Diameter
Female thread gage diameters are measured across the openings in the centers of these disc-like gages. Male thread gages are measured across the flat head ends. They range from less than a quarter inch to over an inch. Highly specialized gages have been developed to be extremely small, almost invisible, and a few more have been developed that are vast in diameter. However, these specialized gages are used only in the most unusual of circumstances. The gages you should be looking at for purchase and for use in factory and industrial settings are those around the half-inch size, many of which will also include 7/16", 3/8", 5/16", etc.
Contact a company like WESTport to learn more.