How come I'm logged in on discuss.area51 but not on area51 itself?

Question

I followed an answer-notification (shown in the top bar on SO) to discuss.area51.stackexchange.com. I was logged in automatically, I could respond to the answer with a comment, and edit my question to clarify.

Then I went to the main site, area51.stackexchange.com. I found I was not logged in; I had the log in link in the top bar.

According to this answer I should have

(...) a single shared account between the two of them.

So how come I'm logged in on the discuss sub-site, but not on the main site?

Answer 1

The auto-logging-in-between-multiple-sites functionality is called universal login. Universal login is implemented for our Q&A sites (which Area 51 Discussions is also one).

However, the Area 51 main site is not based on the same codebase, and does not have universal login. So you will need to log in to Area 51 separately. You will still be linked to the same account, so Dori's post that you linked to is still correct. You just have to log in separately, once for the Q&A sites, and once for Area 51.