According to the Bernstein Medical Center, the Norwood classification published in 1975 is the most widely used classification for hair loss in men and describes seven stages. In the Norwood pattern, two areas of hair loss gradually enlarge until the entire front, top and crown of the scalp are bald.
As WebMD explains, there are many types of hair loss, also called alopecia. Involutional alopecia is a natural condition in which the hair gradually thins with age. Androgenic alopecia, also called male pattern baldness, is characterized by a receding hairline and gradual disappearance of hair from the crown and frontal scalp beginning in the late teens or early 20s.