It is a widely accepted and reproducible classification system for male pattern hair loss (androgenetic alopecia).

[1] Androgenetic alopecia follows a defined pattern of hair loss, beginning with bitemporal recession of the frontal hairline.

The bald patch progressively enlarges and eventually joins the receding frontal hairline.

This measurement scale was first introduced by James Hamilton in the 1950s and later revised and updated by O'Tar Norwood in the 1970s.

The scale is regularly used by doctors to assess the severity of baldness, but it is not considered very reliable since examiners' conclusions can vary.