Three types of skin lines have distinctive patterns that can be helpful in diagnosis:
Dermatomal lines reflect innervation of the skin. They are important in diseases of nerve such as infection of nerve with a virus, or damage to a nerve. The figures here and here show the distribution on the body and the photo here illustrates an infection following a dermatomal line.
Blaschko lines reflect migration of embryonic cells in the skin. The figure here shows the distribution on the body. These are seen in genetic diseases such as incontinentia pigmenti and inflammatory disorders such as lichen striatus.
Langer lines of cleavage correspond to the orientation of collagen fibers in the skin. The figure here shows the distribution on the body. Incisions parallel to these lines heal with less stress on the wound. The smaller secondary lesions of pityriasis rosea follow Langer’s lines as illustrated in Figure 4 here.