I just learned that from “Coupling”. So if a British person asks to see your pants, don’t be bewieldered, be offended!
But what’s a “gusset”?
Actually, it’s “underpants”.
IIRC, the “gusset” is that swatch of fabric that actually covers the Holy-of-Holies…the crotchpiece, as it were. 
Ever hear of “gusset typing”?
oh knickers
Well then, what are “tights”?
Tights are pantyhose.
Well what are knickers?
Knickers are underpant/pants/panties.
It’s all true.
Pants and underpants are the same thing here.
American…British
Pants…Trousers
Pantyhose…Tights
Or alternatively, you can have “wonderpants” or “underchunders”.
It is also worth mentioning that ‘pants’ is used as a colloquial term for ‘inferior or otherwise generally unsatifactory’; examples:
“Have you seen Highlander II? I thought it was pants.”
“Although on paper, the [some car or other] looks to be an all-round good performer, in reality the driving experience is a pile of pants”
Oh and other UK (slang, possibly quite local) terms for underwear include grots and skids.
More to the point, knickers are female underpants. Panties tends to be used for knickers as well.
And whips. And kaks. (again, probably very local)
I have cousins near my age who were born and raised in England. I saw them every summer growing up and as a result I call jeans and work slacks “trousers,” because I was teased unmercifully when I used the word “pants” to refer to the things you put on OVER your underwear.
So, yeah.
Oh, and napkins are diapers. Don’t get them mixed up.
Are you *quite * sure of that?
(diaper -> nappy)
(napkin -> serviette)
I may have been misled…
suspenders --> garters.
I remember the day I learned about “pants” very well. I was walking around Earl’s Court, and some whacko has removed his jeans and was running around screaming at everyone. The police had to be called to haul him away. By buddy promptly called someone on his cell… er, mobile, and told them of a crazy guy running around the street in his pants. I almost corrected him, then I figured it out.
There is one context in which the American use of “pants” is closer to the British–the expression “getting into her pants”–i.e. successfully achieving a situation in which she removes her pants, presumably as a prelude to consensual sex. The way it’s used leaves little room to suppose that a woman referred to in this way was necessarily wearing slacks or jeans.
The expression “pantsuit”, meaning a fairly dressy woman’s trouser and jacket combo, must strike British ears as ludicrously funny.