For years I wondered why so many writers and publishers were choosing to use en dashes (which are the width of a capital N) where em dashes (yes, the width of a capital M) used to be standard.
Em dashes have long been used to separate phrases and clauses—groups of words that contain a subject and a verb but aren’t written as an independent sentence. The dash in the previous sentence between the words clauses and groups is an em dash.
En dashes were normally used to separate a range of numbers (such as 50–100) or to join a compound word to another word or words (such as New York–Chicago).
In typing class, back in the (ulp) 1960s, we used two hyphens in place of em dashes–because typewriters didn’t have em dashes.
Microsoft Word has a feature called “Autoformat as you type,” and one of the choices under “Replace as you type” is “Hyphens (–) with dash (–).” (I don’t use it because I do a lot of HTML formatting in Word, and replacing double hyphens with dashes will ruin some of the code.) But if you use this feature (and I believe it’s turned on by default) and if you type two words with two hyphens between them, Word will replace the two hyphens with an em dash. What the “Replace as you type” menu doesn’t tell you, and what I didn’t learn until this month (September 2016) is that if you type a space, a hyphen, and another space, Word will turn the hyphen into an en dash, as illustrated in the title of this blog post.
My guess is that a lot of writers and publishers have not been choosing to use an en dash in place of an em dash. My guess is that a lot of writers don’t know one kind of dash from the other but have been typing space, hyphen, space where they wanted a dash, and Microsoft Word supplied one, and the publishers left it as written, or, to be precise, as typed by the author and as modified by Microsoft Word.
My solution when editing in Microsoft Word is to search for en dashes with space before and after them (in the Find menu, enter a space, then alt+0150, then another space) and change them to em dashes without space if that’s appropriate or to en dashes without space if that’s what they should be.