A Typographic Subtlety

We have five horizontal lines in English writing. I’m not going to write about the strikeout and the underscore today. The other three are the hyphen, the N-dash, and the M-dash. I’ll skip the hyphen, too, except to say that you shouldn’t use it in place of the two dashes. Unless you’re using a typewriter, where you don’t have a choice.

The N-dash shows a range such as the opening and closing times of a store. 5:00–9:00 for example. An N-dash is the width of a capital N.

The M-dash (width of a capital M) indicates a break of some kind. An interruption, change of thought, or to emphasize a parenthetical idea.

The rules permit you to use an N-dash with spaces instead of an M-dash. But don’t.

Never use spaces around an M-dash—and that leads to my quoted passage. Until today I had never seen anyone put spaces with an M-dash.

In a discovery that raises fundamental questions about human behavior, researchers at the University of Virginia School of Medicine have found that the immune system directly affects — and even controls — our social behavior, such as our desire to interact with others.

This is from an article in a Kurzweil newsletter. Those are M-dashes, and they shouldn’t have spaces around them. They do correctly emphasize the parenthetical remark, though.

Did I forget any rules about dashes? Tell me in the comments.