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Carolyn Trietsch recently defended her Ph.D. in entomology at Penn State. You can view her website or follow her on Twitter @carolyntrietsch.

I’m ashamed to say that I did not know what an en dash or an em dash was until the last few years of my Ph.D. I had used em dashes in my own writing, sure, but I didn’t know what they were called. As for en dashes, I didn’t even know that they existed. It turns out that all of these years, I’ve just been sticking hyphens where they don’t belong.

It wasn’t until after I started learning more about technical copyediting that I found out all I needed to know about hyphens, en dashes and em dashes. The difference between them was one of the simplest things I learned about technical copyediting, yet also one of the things that has stuck with me the most and made a difference in my writing.

For all those in a similar position, here is what I’ve learned:

The Hyphen

A hyphen is used with modifiers that come before nouns. A modifier is a word or group of words that describes a noun. For example, in the phrase “a two-month-long study,” the noun is “study” and the modifier is “two-month-long,” which is hyphenated. Here are some more examples:

“Well-known artist”
“State-of-the-art technology”
“Signal-to-noise ratio”

It’s important to note that hyphens are only used when the modifier comes before the noun. While “a two-month-long study” would be hyphenated, “the study was two months long” has no hyphens because the modifier, “two months long,” comes after the noun, “study.”

Hyphens are also used when family names are joined, such as after a marriage (Ortega-Blanco, Brady-Miller).

Hyphens are typically not used with prefixes (midseason, interspecific, postscript), but they can be when the prefix precedes a capitalized word (mid-November) or number (pre-1970, post-1980). Hyphens are also used where the meaning of the word would be unclear without one (pre-date vs. predate) or when the nonhyphenated word would be difficult to read because of the doubling of letters (cross-species vs. crossspecies; re-emerge vs. reemerge), though for some words this is becoming the norm (as with coordinate, cooperative and microorganism). When in doubt, look it up -- see how others write in your field, and check the style guides and “Information for authors” sections when submitting manuscripts to different journals.

One last note: hyphens should not be used with adverbs that end in “ly.”

Incorrect: “empirically-based model.”
Correct: “empirically based model”

Incorrect: “carefully-worded argument”
Correct: “carefully worded argument”

The En Dash

The en dash or N dash is longer than a hyphen, spanning the length of a capital N (hence its name), and is used to indicate a span or range. Examples include the following: 1–20mg, 10–20 pages, 8–11 p.m., Monday–Friday and January–December.

This rule does not apply when you introduce the range with the words “from” or “between”, such as when you write “from ­­­­____ to ____” or “between ____ and ____.” In these cases, do not replace “to” or “and” with an en dash.

Incorrect: He taught from 1976–1982.
Correct: He taught from 1976 to 1982.

Incorrect: She taught between 100–150 students every year.
Correct: She taught between 100 and 150 students every year.

En dashes are also used to connect words that have equal weight. Equal-weight words could be the same word (cell–cell interactions), related words (blood–brain barrier; oil–water interface, acid–base reactions), or proper nouns, which are capitalized and indicate an individual person, place or organization (Michaelis–Menten equation, France–Germany border).

The Em Dash

The em dash or M dash is longer than the en dash and spans the length of a capital M. Em dashes may or may not have spaces on either side based on the publication and style of writing (American English typically does not put spaces around em dashes, whereas British English typically does). Whatever you do, be consistent!

An em dash is used to indicate a pause or a shift in thought, and can be used to put emphasis on a comment. Consider the example below:

“Museum specimens are fragile––be sure to handle them carefully!”

Em dashes are also used to offset parenthetical matter, such as an author comment or text that clarifies a statement. In this way, em dashes can be used instead of parentheses, commas or colons.

“Several institutions––both national and international––contributed material to this study.”

Now that you know how to use them, how do you make them? This can vary across different systems, devices and word processing programs. Check out this article for a good overview of how to create them on different systems.

If you’re looking for other ways to strengthen your writing, check out these posts from “GradHacker” on tips for better writing in any discipline and tips for teaching and assessing writing. And as always, feel free to leave your comments and questions below.

Until next time -- happy writing, happy holidays and a happy New Year!

Images created by Carolyn Trietsch.

A special thanks to Julie Todd for reviewing earlier drafts of this post.

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