It is a way to measure the extent of male pattern baldness.
The Norwood scale is a way to measure the extent of male pattern baldness, and it is the generally accepted standard when describing hair loss in general.
Norwood Scale - Stage 1
Minimum or no recession of the hairline, therefore no need for treatment. Unless you have a family history of baldness, there is no reason to worry. If there is a family history of male pattern baldness, you may want to monitor the situation closely and decide on the appropriate time for treatment.
Norwood Scale - Stage 2
Triangular and typically symmetrical areas of recession in the anterior temporal area. Hair loss remains in front of a line several inches in front of the ears. The hair falls out and can become less dense in the central front part of the scalp. The first signs of baldness are becoming evident.
Norwood Scale - Stage 3
This represents the lowest level of hair loss considered sufficient to be called baldness according to Norwood. Most of the scalp at this stage has a deep symmetrical recession that occurs on bare temples or only poorly covered by hair. With the vertex of stage 3, the crown is added because it is a common event with age. Hair loss is mainly from the vertex with limited recession of the anterior temporal attachment
Norwood Scale - Stage 4
The recession in the anterior temporal areas is more severe than stage 3. There is a marked lack of hair on the crown. A moderately dense band of hair that extends across the top separates the two areas of hair loss between the temporal front and crown. This band bridges the hair-covered areas on the side of the head.
Norwood Scale - Stage 5
In phase 5 the loss of hair in the vertex region is still separate from the anterior temporal region, but the division is much less distinct. The band of hair that extends through the crown is visibly narrower and thinner. The hair loss at the vertex and the frontal regions are greater. When viewed from above, stages 5 to 7 show the remaining hair on the sides and on the back as a particular horseshoe shape.
Norwood Scale - Stage 6
The bridge of hair that once crossed the crown was lost with only a few sparse hairs. The frontal and vertex regions are now united in a single area. The hair loss on the sides has further extended.
Norwood Scale - Stage 7
This is the most advanced or severe form of hair loss. Only a narrow band of hair in the shape of a horseshoe survives on the sides and back of the fine c and less dense than before. At the back of the head the hair is sparse with a semicircle on both ears.