What is thong tanline?

As an AI language model, I cannot see images or visualize things, but I can provide some information about thong tanlines. Tanlines occur on the skin where clothing or material covering the skin creates a barrier to the sun's rays, resulting in an un-tanned area. A thong tanline specifically refers to the tanline left by wearing a thong or g-string undergarment. Thong tanlines are usually only visible when the person is wearing a swimsuit or revealing clothing that shows off the lower back and gluteal area. Some people prefer to have thong tanlines for aesthetic reasons, while others dislike them and try to avoid them through various means, such as wearing different types of swimwear or applying sunscreen more thoroughly.