A tan line is a visually clear division on the human skin between an area of pronounced comparative paleness relative to other areas that have been suntanned by exposure to ultraviolet radiation. The source of the radiation may be the sun, or artificial UV sources such as tanning lamps.
Tan lines are usually an inadvertent result of a work environment or of recreational activities, but are sometimes intentional. Many people regard visible tan lines as un-aesthetic and embarrassing, and seek to avoid tan lines that will be visible when regular clothes are worn.
A biker's tan (shown here) is a tan line that goes up 3/4 of the leg, where lycra shorts would normally begin to cover. Depending on the activity, the inner side of the arms may be paler than the outer side. Unless the biker uses cycling gloves made to allow tanning, the area on the back of each hand will usually not be tanned. ‪#‎realgirlsthatride‬


via drunkcyclist.com