A few words of advice for our American friends.

Hardly any woman in the U.S. uses any other word but panties to describe their underpants. Briefs describe a certain style and a friend of mine who grew up in the South told me she and her friends called them "britches" which I associate with jeans. It seems fair to use panties for U.S. settings and knickers for the U.K.

lesliejones
The American terms for underpants I mentioned are all older or regional , agree they are pretty much not used now, except for the expression "drop your drawers" which can be unisex.
" Briefs" while it can refer to female undergarments, more often refers to men's underpants, as in briefs(aka jockey shorts) as opposed to boxers.
And definitely, knickers for British stories, panties for American.

Is there even a British corresponding term to "panty lines"?

I suppose you could say "knicker lines", Glagla, or "vkl."
While on that same thread of thought, take the word "shorts." Means either abbreviated trousers worn as an outer garment by either men or women- or men's, but not women's underpants.
"Thongs" can either be swimsuits or underpants. And likewise "bikini" although for underwear we usually say "bikini panties."
And why is another type of skimpy panties called "boyshorts?"

I had never heard the term boyshorts. I checked the Almighty Internet and it looks much like boxers or briefs to me. Is this a US term that is common?

Do you say bikini bottom by the way? Meaning the bikini then of course.

Do people really say that they wear a thong in Britain because they want to avoid disfiguring knicker lines? Or do you say panty lines as well, even though it's not usually used for the underwear?

What's vkl. short for?

What would be the most common, to wear a thong bikini or a string bikini, or is there an actual difference?

Listen here, Limey: when you can weather a Texas summer, then and only then can you talk down to us!

Vkl would stand for visible knickers line if you hypothetically wanted an equivalent in British English for vpl.
I have heard the explanation that thongs are worn to avoid panty lines. Could be true, but expect a lot are worn because they are considered fashionable sexy underwear.
A bikini by definition is a scanty two piece swimsuit for women. That's the original meaning, but of course it came to be applied to skimpy panties that resembled bikini bottoms in the swimsuit.
The "tanga" or string bikini originated in Brazil in the mid 70s. It exposed a lot but usually not all of the bottom cheeks. Not sure when later the thong bikini which usually exposed the entire bottom emerged. And then there are the g-string bikinis. Now it seems thong and string are usually more or less interchangeably in English.
A a bit later the "frio dentale" or floss bikini also originated out of Brazil, which pretty much defines the minimum that can be worn short of nothing at all.

"Scanties" and "Skimpies" are two terms for barely-there underwear that were apparently popular in the 1920s-1930s, but seem to have fallen out of use.

Seven pages of comments on this topic which I started back in 2011 and to date, only one comment on my latest story - The School Journey (plug) Oh well I suppose I can wait until 2025 for the comments on my story to mount up.

Hotspur
Complain, complain, complaint - is that all we get around here (even if tongue-in-cheek)?