<p>math.stackexchange.com has long been here, and mathoverflow.net, which has functioned for, I suspect, about five years, only recently got absorbed into stackexchange.</p>

<p>The vast majority of questions appropriate for math.stackexchange.com are <b>not</b> appropriate for mathoverflow.net, but perhaps any question appropriate for mathoverflow.net would also be appropriate for math.stackexchange.com.  It has long been customary that a question posted math.stackexchange.com that gets no attention there may be cross-posted to mathoverflow.net <b>provided</b> that they're the sort of questions appropriate for the latter forum, and also provided that one states that one is cross-posting, with links in both directions.  This became customary long before mathoverflow.net recently became a part of stackexchange.</p>

<p>Thus it is incorrect to say that <b>all</b> stackexchange forums consider it <b>always</b> wrong to cross-post.</p>
