Interesting point about the tan lines from long before the "UPF" marketing of clothing. While there was clearly some UV filtering going on to produce the tan lines, I suspect that the presence of multiple layers of cloth in bathing suits etc. improve most any fabrics' UV filtering capability. So maybe the "trick" per se is in being able to achieve effective UV filtration in very thin/light/breathable fabrics.
Then again...maybe not! Just a thought.
