As correctly pointed out by ASimpson, there are classification of fits which are application specific. He lists the metric version, imperial are equivalents noting 1.0 inch = 25.4 mm. There are several good references on this, look to the web. But the Machinist Handbook does discuss this, at least in the 23rd edition.
How does Dhea work? DHEA is a parent hormone produced by the adrenal glands near the kidneys and in the liver. In men, DHEA is also secreted by the testes. It is changed in the body to a hormone called androstenedione. Androstenedione is then changed into the major male and female hormones.
Scientists don't know everything DHEA does. But they do know that it functions as a precursor to male and female sex hormones, including testosterone and estrogen. Precursors are substances that are converted by the body into a hormone. DHEA production peaks in your mid-20s. In most people, production gradually declines with age.