An em rule (em dash) is used as a parenthetical pair to convey digression or an associated thought; or as a single punctuation mark to convey an aside or afterthought.
Example: use em rules—those are the longest of the three horizontal lines—to convey a stronger break than would be covered by a pair of commas.
Example: These examples are quick demonstrations of em rules—I hope people find them helpful.
An en rule (en dash) is used to signify a range, an association between two words, and the omission of a letter.
Example: Open Monday–Friday.
Example: The Hawking–Penrose theory.
Example: Don’t bloo– – swear!
A hyphen is used to signify that two (or more) words have a combined meaning. The use of hyphens is more complicated and contains greater disagreement than does the use of en- and em rules.
Example: The quick thinking-runner fell in the hole; the quick-thinking runner jumped over the hole.
Example: post-industrial Britain.
Example: head north-north-east to the spot marked with an X, where you will find one-half of the treasure.
The ability to dash properly is a useful skill—especially when working with running text. Still, I often see otherwise dashing material that’s been dashed by being dashed off with a wrong dash. It’s a dashed shame.
A grammatical ‘dash’, as the Concise Oxford English Dictionary puts it, is ‘a horizontal stroke in writing, marking a pause or to represent omitted letters or words.’ This refers to two separate punctuation marks: the en rule and the em rule. Note that a ‘dash’ does not mean a hyphen; a grammatical dash is not just any horizontal line.
There are many variations in style when dealing with hyphens and the two dashes; Freelancealot.co.uk has chosen to follow the style recommended by Oxford’s New Hart’s Rules. However, my experience is that this style is not the most common in British publishing—look at some newspapers and judge for yourself—so I have mentioned other commonly used styles where it seemed helpful. Though Freelancealot will consider working to any grammatical style supplied by a client, we prefer the New Hart’s Rules guidelines for en rules and em rules as they allow for the enhanced conveyance of emotion and precision through the use of text—and that is what we strive to achieve, 9–5, Monday–Friday, and sometimes over the weekend too.
Further Reading: Em Rule (aka Long Dash, Em Dash)