B: Braies

Braies (with hose) on the man in blue, and just braies
on the man to the left of the pillar

There are many (many) terms out there for underwear, and I eventually settled on braies for men's underwear in my fantasy monstrosity -- mostly because it fits the language style I wanted (antiquated but not intimidatingly so) and it's another thing for me to argue with SpellCheck about (not that I needed help with that.)

Costume research is one of those things that looks like it should be simple, but turns out to be complex because there is so much information out there and much of it is contradictory or misinformed by the heavy influence of Hollywood and modern sensibilities. When it comes to medieval-style clothing, I've found serious re-enactors and SCAers to be a good resource (though it can take a lot of slogging through bad links.) They can also offer insights into sewing, wearing and working in the outfits. 

As an example of tricky research: medieval women's underwear. There's near-universal agreement that women wore a shift/chemise under their dresses, which was essentially a lightweight dress itself. Over time, it evolved into the slip (and who wears slips anymore?) But that's where agreement ends.

Eventually, I had to conclude that there's no solid evidence either way on the question of whether medieval women wore underpants/panties at all. There's no solid evidence of breast-supporting devices until corsets came along, either, though binding (in the form of a bandage-like wrapping) does get mentioned now and then.

So I actually went with the slightly scandalous option of my women wearing a shift under their dresses and nothing else unless it was that time of month (because good Lord, what a mess if they didn't.) They'd wear woolen hose when it's cold -- that's just being sensible.

How do you dress your fantasy characters?