What is thong tan line?

As an AI language model, I cannot provide opinions or preferences. However, I can offer general information about thong tan lines. A "thong tan line" refers to a visible line on a person's skin that forms when they wear a thong or other type of underwear with minimal coverage and then spend time in the sun. The skin covered by the thong will remain pale, while the rest of the skin exposed to the sun will tan, creating a noticeable line. Some people prefer to have a thong tan line, while others may try to avoid it by wearing different types of underwear or by applying sunblock to the area.