Em dash

Three marks share a key on most keyboards and none of them are interchangeable. The hyphen (U+002D) joins: well-known, part-time, and the break at the end of a hyphenated line. The en dash (U+2013, –) spans: 2010–2014, London–Paris, pages 12–18. The em dash (U+2014, —) breaks: it marks an interruption, an aside, or a change of direction mid-sentence, and it is the only one of the three that can do the job of a comma, a colon, and a pair of parentheses depending on how it is used. The names come from the metal: widths of an en and an em, the em being the square of the type size, which is where the rule of thumb about the letter m comes from.

Punctuation around it is a house style rather than a rule. American style, and Chicago, set the em dash tight against the words either side. British and Canadian practice, and most newspaper style guides, use a spaced en dash for the same job. Pick one and hold it, because the two together in one document read as carelessness. Both are more readable than the typewriter substitute of two hyphens, which still survives in plain-text formats and in editors that will autocorrect it if you let them.

The typographic details are small and worth knowing. Set an em dash with no spaces and most browsers will happily break the line after it but not before it, which is usually what you want; a spaced en dash breaks on either space, so guard the leading one with a non-breaking space if a dash stranded at the start of a line bothers you. Some faces draw the em dash short of a full em and others draw it with side bearings so that a run of them does not close into a solid rule, which is why --- looks like three dashes in one typeface and a continuous line in another.

This site never uses one. Long-dash-heavy prose has become the most quoted surface tell of machine-written text, and a reference whose credibility rests on being written by someone cannot afford to read like output. A comma, a colon, a full stop, or a pair of parentheses covers every job the mark does, and the constraint is a good editor: most sentences that reached for one were two sentences. The specimen above is the only place on the site where the glyph itself appears, which feels like the right amount of respect to pay a mark that has done nothing wrong.