How much reputation do you need to participate in chat?
It's an easy answer, right? You need at least 20 reputation to participate in chat.
/privileges/chat even tells us so.
But for which chat site(s) is that?
Because we have at least three chat domains:
The confusion is compounded by support requests like this one where the user had enough reputation but couldn't access chat, and this one where the user gamed their general StackExchange rep and could participate in chat.
And all the above is made a little bit more unclear because once you earn the association bonus then you can just register with the associated site and this is no longer an issue.
So what's the correct answer for how much reputation and where does it need to be from in order to chat?"
Is the correct answer "You need to have at least 20 reputation on a site in the domain that the room belongs to in order to chat in that particular room?" And if that is correct, is that for all chat rooms within a given domain or is there an additional per room check tying back to the owning site of the room?
Wait, what?! Who cares?
Well, it's easy to shrug this off once you've earned 200 rep. However, when you're trying to provide guidance to other users about inviting them into chat, it's important to know just how much reputation and from where a new user needs to have in order to participate in chat.
Sidenote: This question is closely related as it explains what additional privileges you get in chat with increased reputation or room ownership. But it doesn't address where the precious reputation needs to be earned.