U.K. and U.S. Dashes . . . Don't Mix Them Up

    In the U.K., the standard convention for introducing apposed sentence content with a less formal appearance than a colon and for presenting internal sentence content in a more emphatic way than parentheses is the open en dash. For the same purposes here in the U.S., we use the closed em dash.

    U.K., open en dash
        I don’t know – I don’t think anybody does.
        I don’t know – I don’t think anybody does – but I can find out.
    U.S., closed em dash
        I don’t know—I don’t think anybody does.
        I don’t know—I don’t think anybody does—but I can find out.

    The open en dash is, not unexpectedly, endorsed by no less than the big gun of British English usage, the University of Oxford Style Guide, but the American em dash is nevertheless widely used in the U.K. because of the increasing adoption of American-style punctuation in general. Conversely, here in the U.S., there is essentially no reciprocal affection for the U.K.’s spaced en dash: it’s eschewed by every U.S. style guide in favor of the closed em dash, so it draws attention when encountered in written American English. It’s a free country, of course, and you’re welcome to use it if you want to and don’t mind that it will usually be unfamiliar and therefore offputting to American readers, as will be U.K.-style dialog punctuation.

    If you're writing for an American readership, use American-style punctuation, including, for the love of God, dialog.