Today I received a request for a “Writing Glitch” post dealing with how to use dashes correctly. Alas, in the examples I’ve been gleaning from Pinterest, Facebook, and other sites, sentences where a dash should be used are conspicuous in their relative absence.
When I talk about dashes, I mean the em dash unless otherwise specified. (An en dash looks a lot like a hyphen, although it’s used quite differently. Back in my day, dashes were dashes, and hyphens were hyphens, and furry little critters from alpha Centauri were… Never mind.)
Anyway, here are a few things The Chicago Manual of Style (Sixteenth Edition) says about dashes:
Using dashes or parentheses (6.92): “Parentheses — stronger than commas and similar to the dash — are used to set off material from the surrounding text. Like the dash but unlike commas, parentheses can set off text that has no grammatical relationship to the rest of the sentence.”
Em dashes instead of commas, parentheses, or colons (6.82): “Em dashes are used to set off an amplifying or explanatory element and in that sense can function as an alternative to parentheses, commas, or a colon — especially when an abrupt break in thought is called for.”
Em dashes to indicate sudden break (6.84): “An em dash or a pair of em dashes may indicate a sudden break in thought or sentence structure or an interruption in dialogue. (Where a faltering rather than a sudden break is intended, suspension points [ellipsis …] may be used; see 13.39.)”
The main thing to remember is that dashes indicate abruptness: the sentence changes direction suddenly, or the speaker is interrupted.