I have a confession to make.
Until recently, I did not know the difference between a hyphen (-), an ‘en dash (–),’ and an ‘em dash (—).’ I did not even know that the latter two had names. If you had asked me a year ago, I would have said that these three horizontal lines were interchangeable.
They are not.
I am somewhat comforted by the fact that it appears that many scientists, presumably as well educated as I am, also do not seem to know the difference between a hyphen, an en dash, and an em dash.
Fortunately for all of us, the rules regarding the use of these dashes are reasonably straightforward.
The smallest of the dashes, the hyphen, has two uses. The first use involves joining the parts of a compound adjective. A typical compound adjective that appears in the scientific literature is ‘well-known.’ Others are ‘first-line,’ and ‘full-length.’
Strunk and White caution that one should not use a hyphenated compound word when one single word will suffice. The example they give is ‘waterfowl’ instead of ‘water-fowl.’
The problem, of course, is how does one know if there is a hyphen or the two words should be one? Strunk and White recommend using a dictionary. For scientific writers, a better choice is Google Scholar. If you are tempted to use a word combination, chances are someone in the literature has used the combo before you. Just enter the two (or more) words into the search box and scroll through the hits.
What you find may confuse you. In some cases, authors use words separated, some use a hyphen, and some combine two words into one. You may have seen this with the words ‘wild’ and ‘type.’ According to Google Scholar ‘wild-type,’ ‘wild type,’ and ‘wildtype’ all appear in the literature. Which one do you choose? My suggestion is to pick whatever looks right to you.
A Dash of Common Sense
Strunk and White warn that one must be careful with the hyphen. They tell the story of an unfortunate consequence of the otherwise appropriate use of the dash. The city of Chattanooga, Tennessee had two newspapers, the News and the Free Press. When the two newspapers merged, they became The Chattanooga News-Free Press. The placement of the hyphen made it appear that the paper was ‘news-free,’ in other words, devoid of news.
According to Google Scholar ‘wild-type,’ ‘wild type,’ and ‘wildtype’ all appear in the literature. Which one do you choose? My suggestion is to pick whatever looks right to you.
To avoid similar mistakes, Strunk and White advise not asking the poor hyphen to do too much work. I believe that’s sound advice.
But Wait, There’s More!
In scientific writing, hyphens are often needed to express age, time and distance. For example, a rat that was born one week ago is a ‘one-week-old’ rat. You do not want to say that the animal is a ‘one-week old rat.’ The latter suggests that the rat has been old for a week. In histology and pathology results, authors will often use a hyphen in a description of the thickness of sections. For example, ‘4-µm sections were cut using a microtome.’ Incidentally, it is not necessary to say ‘4-µm thick sections’. The word thick is a redundancy (or as I like to call it, a pizza pie.) If you were to write ‘4 µm sections,’ the reader might interpret it to mean that there were four sections and each was one micron thick.
The bottom line (see what I did there?) is that the hyphen is there to help you avoid confusing the reader.
The En Dash
The slightly longer en dash is so named because its length is the same as capital ‘N’ in a particular font.
According to the Punctuation Guide, an en dash ‘is used to represent a span or range of numbers, dates, or time. There should be no space between the en dash and the adjacent material. Depending on the context, you read the en dash as “to” or “through.’
For example, ‘the Golden State Warriors are the 2017–2018 NBA champions.
‘The animals were fed daily 8:00 a.m.–9:00 a.m.’ If you will be using words such as ‘between,’ ‘during,’ or ‘from,’ do not use the en dash.
Correct: The age range was 17–80 years.
Incorrect: The age range was from 17–80 years.
It Gets Worse
There is more bad news about the en dash. The mark does not appear on a standard keyboard. I am writing this in Microsoft WORD on a MacBook Pro Air with a standard US QWERTY keyboard. To find the en-dash, I need to go to Insert–Advanced Symbol–Special Characters, highlight the en dash and hit Insert. This is a pain. So, I created a shortcut. Command-dash is now my en dash. I recommend you do likewise.
The Em Dash
As with its sister, the en dash, the em dash is the same length as a letter, in this case, capital ‘M.’
I am going to make this easy for you: do not use the em dash in scientific writing. There is no reason to use it. If you like, you may now stop reading. For the rest of you who are still interested, here is what the Punctuation Guide says about the em dash:
The em dash is perhaps the most versatile punctuation mark. Depending on the context, the em dash can take the place of commas, parentheses, or colons—in each case to a slightly different effect.
Notwithstanding its versatility, the em dash is best limited to two appearances per sentence. Otherwise, confusion rather than clarity is likely to result.
Like the en dash, you use the slightly longer em dash without spaces. Your word processor likely agrees. In MS-WORD, for example, if you type two hyphens immediately after a word, it will automatically change to an em dash. There is no such short-cut for the more common en dash.
Do yourself and your editors a favor: learn the rules regarding dashes. It will make their job easier and will make you look like an English-language superstar.