I didn’t know what an “en” dash was until I started working as a copy editor. Somehow I’d never even noticed them before. I knew when to use hyphens and em dashes, but the en dash was so confusing, no matter how many times I read the official description in the AMA Style Manual:

“The en dash shows relational distinction in a hyphenated or compound modifier, 1 element of which consists of 2 words or a hyphenated word, or when the word being modified is a compound.”

I understood how to use it with a hyphenated modifier, but the compound modifier tripped me up. I was overthinking it, spending too much time debating what the modifier was, and placing the en dash in the wrong place. Then someone explained to me that you use it when a group of words represents a single idea and it started to make sense:

• US army–enlisted population
• Geographic Information Systems–based measures
• gene-dose–dependent manner
• B-amyloid–negative group
• apolipoprotein E–related genetic susceptibility
• estrogen receptor–negative breast cancer

In all of these cases, 2 or more words together represent the single thing (eg, US army, apolipoprotein E), which is your cue to use the en dash before or after the phrase.

Some style manuals recommend using the en dash for ranges, scores, or values that are related, but the AMA Style Manual reserves them for the hyphenated or compound modifier only.—Tracy Frey