Mishnà su Levitico 13:40

Scalp baldness (karahat) or forehead baldness (gabahat) may become unclean in two weeks and by two signs: by quick flesh or by spreading. What constitutes baldness? If one ate neshem or smeared himself with neshem or had a wound from which hair can no longer grow. What is the extent of scalp baldness? From the crown sloping backwards to the protruding cartilage of the neck. What is the extent of forehead baldness? From the crown sloping forwards to the region facing the hair above. Scalp baldness and forehead baldness cannot be combined, nor is spreading from one to the other effective. Rabbi Judah says: if there is hair between them they cannot be combined, but if there is none they must be combined.