The term<unk>  refers to the hair that grows on the abdomen of humans and non-human mammals, in the region between the pubic area and the thorax (chest). The growth of<unk>  follows the same pattern on nearly all mammals, vertically from the pubic area upwards and from the thorax downwards to the umbilicus (navel). The<unk>  of non-human mammals is part of the pelage or fur.
Before puberty, the<unk>  region of both males and females is covered with very fine vellus hair. In response to rising levels of androgens (mainly testosterone) during and after puberty, the skin of the abdomen begins to produce coarser, longer and more pigmented hair (terminal hair). This process affects primarily men. Initially hair grows in a vertical line from the pubic area up to the navel and from the thorax down to the navel. Slang terms for this line of hair include "happy line", "happy trail", or “treasure trail”. Although the development of<unk>  normally begins during puberty it may also start later, between the ages of 20 and 30. With some men, the<unk>  will stay within a clearly defined vertical line, but in others, terminal hair will appear laterally as well as vertically, particularly in the area around the navel. This lateral spreading can continue into middle age.<unk>  (particularly the "treasure trail") often signifies young teen men stepping into adulthood, as it represents that their bodies are changing.
Some women may develop a small line of hair from the pubic area up to the navel. According to the female sex image in some cultures this is seen as desirable, and in others it is seen as unattractive. Excessive<unk>  on women, following the male pattern, is called hirsutism.
Various studies of caucasian subjects have documented four general patterns of pubic and<unk>  including:
Horizontal - Characterized by upper surface of pubic hair terminating in a horizontal line with no hair extending to the abdomen.
Sagittal - Resembles the first but with the addition of a narrow vertical band of hair extending from the pubic hair towards the navel.
Acuminate - Characterized by a tapered, inverted 'V' pattern extending upward from the pubic hair. Upper limit may end below the navel, at the navel, above the navel or near the chest. (Example shown in accompanying photograph.)
Disperse (or quadrangular) - Hair is distributed broadly over the abdomen without forming a discrete geometric pattern.
In Zickler's study (see citation below) the horizontal pattern was most common in females with an incidence of about 80 percent. This pattern was also common in males younger than 25 with an incidence of about 40 percent. The sagittal pattern occurred in about 17 percent of females and 6 percent of males. The acuminate pattern occurred in about 55 percent of males and occasionally in females. The disperse pattern occurred in about 19 percent of the males studied.
Abdominal hair
Nasal hair
Pubic hair
Urogenital folds
Pus
Supernumerary nipple
Pterion
Penis enlargement