I was searching for info on a network issue and accidentally *cough* (I swear!) found out, that Wikipedia actually is home to an article on Visible Panty Lines. What does this teach us? Well, never rely on acronyms for your google-fu.
And secondly: The guy who wrote this article is only surpassed in greatness by Rudi Gernreich (look it up yourself).

Ah, btw: here is the article:
https://en.wikipedia.org/wiki/Panty_line