Dash v. Hyphen

In lots of scientific writing—both published and unpublished—I see authors use hyphens where they intend to use an em-dash. Besides the regrettable effect of proclaiming that you do not understand the difference, more importantly, this mistake often leads to ambiguity and confusion. Here’s a brief summary that will help you use hyphens correctly.

First, you have to be aware: there are 3 different symbols that all look like little lines: the hyphen (-), the en-dash (–), and the em-dash (—). They differ in width, and they mean different things! Many people use a hyphen for all 3; don’t be one of those people. Here’s what they mean:

hyphen: used for compound words. Example:well-oiled
en-dash: width of the letter n; used as the symbol for ranges. Example:7%–9%
em-dash: width of the letter m; a punctuation mark to set apart parenthetic statements. Example: The gene expression—after normalization, of course—showed enrichment in the…

I have yet to see a case where using a hyphen (-) in place of an en-dash (–) causes confusion, so I don’t worry too much about en-dashes. The biggest source of confusion comes from using a hyphen (-) where you intend to use an em-dash—and this happens all the time! Imagine that last sentence with the em-dash replaced with a hyphen:

Big problem, no? If you don’t want to go to the work to use an actual em-dash character, it’s commonly accepted to use two hyphens together (–) to represent an em-dash.
If you do want to create the actual en- or em-dash glyphs, here’s a table that shows you how to create them in different environments: