Just out of curiosity, why "ud"? It seems like any two letter subdomain works in the same way.

I have no idea what ud is, I use old.reddit.com

Honestly, I think that was just one I bookmarked at some point and there it is in my history ;)

It's actually old.reddit.com but they probably have some sort of a wildcard rule.

This is probably it : any two letter combination seems to work.

It was (originally) for localization: ja.reddit.com would (for example) give you a Japanese interface. Eventually they realized what a horrible idea l10n is for a community-oriented site, and dropped the translations, but kept the `lang` that it would apply attributes. Eventually subreddit moderators figured out that you could use this to provide multiple variants of the per-subreddit CSS, such as dark modes, custom filters, and other silliness[0].

[0]: For example, see /r/CrappyDesign with (https://old.reddit.com/r/CrappyDesign/) and without (https://of.reddit.com/r/CrappyDesign) Comic Sans.

Testing it out I found something interesting. You can put in subreddit.reddit.com and it redirects to reddit.com/r/subreddit. But any 2 char prefix works sends to old.reddit.com (a.reddit.com -> reddit.com/r/a)

e.g. hackernews.reddit.com -> reddit.com/r/hackernews

Why does this subreddit exist?

> hackernews.reddit.com -> reddit.com/r/hackernews

> Why does this subreddit exist?

Considering the prevalence of the rebuke "HN is not reddit" on HN, probably for people who want to talk in a more reddit-like style.

Thanks!

Other than "np", as in "np.reddit.com", which is used for "no-participation mode", I think the other two-letter sub-domains are used for language codes. I think at one point in reddit's history doing things like de.reddit.com could give a german site, and so on. I'm not sure what "ud" would refer to, but I assume it's similar.

np is implemented as a language code. Subreddits have to change their stylesheets for that language for it to work.

Maybe "undefined"/"undetermined"?

Try https://reddit.com/.compact