Thanks. What I meant to ask was, given X is the same topic, what is the difference between X overflow and x.stackexchange.com? That is, if there is a general rule.
Perhaps the answer is the same: It's just age. But then if X Overflow already existed, why create a redundant x.stackexchange.com?
That is interesting. I can see the logic there, mostly I'm just impressed that both sites maintain a critical mass of activity, since (at least in recent years) SX seems to require a pretty high level of engagement to get out of perpetual beta.
Does anyone know of any other cases with two similar but distinct SX communities?
Math Overflow was created as a Stack Exchange 1.0 site, which means essentially that SE hosted the software, but everything else was done by the users that initiated the site.
A few years ago, SE stopped supporting the 1.0 sites, and Math Overflow decided to join the Stack Exchange network. They're still somewhat special, e.g. they have the right to leave the SE network again, if they would want to.
At least on the math side, MathOverflow is generally for research level questions, and math.stackexchange is for any level math, often easier, questions.
