Definitely a thong tan line for me. For the past 2 years I have worn my Skinzwear M25U thongs exclusively while tanning. Love their tiny straps and small triangle rear that make for some of the sexiest tan lines in my opinion. There are a few pictures of my tan lines in my profile. There are more if interested.
https://i.vgy.me/knKVFT.jpg
https://i.vgy.me/PtTI0a.jpg