Comment by dragonwriter

There are British style manuals (e.g., the Guardian’s) that prefer em-dashes for roughly the same set of uses they tend to perferred for in US style guides, but it is mixed between em-dashes and en-dashes (both usually set open), while all the influential American style guides prefer em-dashes (but split, for digressive/parenthetical use, between setting them closed [e.g., Chicago Manual] and open [e.g., AP Style].)