Hyphenation and correct use of dashes

by Geoffrey Hart

English writing has many strange characteristics, but one of the most unusual is its use of horizontal lines called dashes. These serve a variety of purposes, and in this article I’ll teach you those purposes and how to easily type these characters.

The first and most common symbol is the hyphen (-), which is the shortest of the three dashes. The goal of the hyphen is to join two or more words to clearly indicate that they function as a single unit. One use of the hyphen in this manner is when your word processor breaks a word into two parts at the right margin of the page; the hyphen indicates that the two parts of the word belong together. This use of the hyphen is an automatic feature of your word processor, so you don’t need to worry about it or learn to use it; the journal that publishes your paper will decide whether it’s necessary to use this kind of hyphenation.

The most important use of the hyphen is to create what is called a compound adjective. An adjective is a word that describes some characteristic of a noun (a person, place, or thing). For example, if you are describing a rice gene, gene is the noun (the thing you are describing) and riceis the adjective that defines what type of gene. A compound adjective occurs when you combine two or more words that describe the same noun; the hyphens then indicate that all of these words work together to describe that noun. For example, you can state that a tree is 10 m tall, or you can describe a 10-m-tall tree. As you can see, the hyphen is necessary when the adjectives come before the noun. They are unnecessary when they follow the noun because the meaning is clear without any further clues. This is also why English words that end in “ly” (mostly adverbs, which describe the characteristics of a verb, which represents an action) don’t require a hyphen. There is rarely any possibility of confusion that would be eliminated by adding a hyphen.

The medium-length English dash (–) is called an “en” dash because it is the width of the letter “n” (pronounced en) in a font. It is commonly used as a minus sign, and although there are subtle differences between the two symbols, in practice those differences are only meaningful to graphic designers who specialize in typography. Like a hyphen, en dashes are also used to join two words that are connected or related, but not to suggest that the two words function as a compound adjective. That is, the two joined words often function as nouns rather than adjectives. For example, in the phrase win–win solution, the meaning is that both people involved in the situation can win; it does not mean that you are using the first “win” to describe a variety of the second “win”. Similarly, cost–benefit analysis means an analysis of both the costs and the benefits; it does not mean a specific type of benefit called a cost benefit.

The en dash can also be used to form compound adjectives in which one part of a compound is two words long. For example, a problem caused by Microsoft Word could be described as “a Microsoft Word–caused problem”. Because this point is a bit subtle, you may find it easier to change the word order: “a problem caused by Microsoft Word”. You could also use only hyphens to replace all spaces: “a Microsoft-Word-caused problem”. That’s not strictly correct, but the meaning is clear, and a journal’s copyediting staff will fix this if they consider it a problem.

En dashes are also used to indicate a range, as in the case when you describe a 1970–2010 study. Although the meaning is clear, I prefer to use the word “to” to indicate ranges because of a common problem in science writing: the presence of negative numbers. In a case such as –2–1, it’s not immediately clear that this is a range from –2 to +1. Writing “–2 to 1” eliminates any possibility of confusion. Similarly, –3– –1 is unclear because without the space that I added to separate the two dashes (so that the separation is clearly visible), you would have to look closely to determine that the range was from –3 to –1, and not from –3 to 1 but with the two numbers separated by the wrong type of dash. Where such confusion is not possible, such as in ranges of positive integers, there is no reason not to use an en dash to indicate a range.

Note: Do not use the symbol “~” (which appears to the left of the number 1 on an English keyboard) to indicate a range. This symbol usually means “approximate”, though the symbol ≈ is a better choice to communicate that meaning.

The longest of the three dashes (—) is called the em dash because it is the width of the capital letter M (pronounced em) in most fonts. It is used primarily to indicate a break in a sentence where you have inserted a parenthetical comment—an explanation or note that provides additional information but that is not crucial—as I have done in this sentence. The phrase is referred to as “parenthetical” because you could replace the em dashes with parentheses, which are also called round brackets: ( and ). Although this is a legitimate form of punctuation, it is less commonly used in science writing than in other forms of writing, and you should generally just use parentheses or rewrite the sentence to avoid the need for dashes or parentheses.

The hyphen (-) character is present on all English computer keyboards at the top right side of the keyboard, towards the end of the row of number keys, so you can type it directly. The other dashes are not present on the keyboard, but most word processors let you type them using simple keyboard shortcuts.

En dash (–):

• In any Windows word processor, hold down the Alt key and type 0150 on the numeric keyboard.

• In Microsoft Word for Windows, press Control+Alt+[hyphen]

• In any Macintosh word processor: press Option+[hyphen]

Em dash (—):

• In any Windows word processor, hold down the Alt key and type 0151 on the numerical keyboard.

• In Microsoft Word for Windows, press Control+Shift+Alt+[hyphen]

• In any Macintosh word processor: press Option+Shift+ [hyphen]