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.
While this hormone has strong ties to fertility issues, a decrease in DHEA can also lead to other conditions later in life, such as Alzheimer's disease, depression, osteoporosis, and heart disease. DHEA supplements are synthesized in a lab and typically sell for less than $20 per 100-count bottle at vitamin stores.