But underwear are still underwear, regardless.

The first is just a T-shirt.

a-shirts are the only undershirts I have.

But are they fun to wear?

The a-shirts I normally see worn outside are a heavier weight cotton than the ones meant as undershirts. I see it as the equivalent of a woman going outside in a sports bra - ultracasual, but not quite "in underwear."

The first is just a T-shirt.

To you. Not to all the guys at my work wearing them as undershirts right now, though.

Underwear is just underwear, but no one is going to ask me "why not just say underwear instead of panties?" are they? Except I've been led to understand that underwear here doesn't include bras, so I don't know what that's about. If I want to specify panties, why wouldn't I?

The a-shirts I normally see worn outside are a heavier weight cotton than the ones meant as undershirts.

The ones I wear are definitely undershirt weight, which is why I wear them doubled.

Ordinarily, since I haven't gotten out of bed yet, I'd be wearing an a-shirt but I'm wearing my retina burning jammies today- green and purple plaid bottoms with a purple and pink tie-dye tee shirt.

I go back and forth on the use of the term, myself. And indeed, sometimes it depends on who I am talking to and their level of sensitivity, which I know is a bad way to do it!!

YAY DREW WITHOUT TUBES (except the intertubes, because we want him to have those)

Why aren't they just undershirts?

For me, because I am usually talking to costume people or actors, I need to make a distinction between a t-shirt and an a-shirt because that is usually a choice made by the designer (even if worn under shirt).