Hyphens (-) indicate conjunction. There are three main uses.*
To distinguish between homographs (to re-dress means to dress again, but to redress means to set right).
To link certain prefixes with their main word (non-Western, mid-year). NB There is a clear trend to join both elements, particularly in American English in all varieties of English (subsection, nonlinear). A hyphen is more likely when the adjacent letters are the same or are both vowels (non-negotiable, pre-industrial), or where a word is uncommon.
To link related terms in compound adjectives and adverbs: (face-to-face discussion, hand-fed turkeys).
Disambiguation. (little-celebrated paintings isn't a reference to little paintings).
Before versus after the noun. Many compound adjectives are hyphenated before the noun (a light-blue handbag), but not after (the handbag was light blue), but where it might be unclear, the hyphen may be used after the noun: the turkeys were hand-fed).
-ly adverbs. Normally don't use a hyphen after an -ly adverb (a wholly owned subsidiary).
Hanging hyphens.two- and three-digit numbers.
Units versus symbols. Hyphenate values and units used as compound adjectives only where the unit is given as a whole word: a 9-millimetre gap, but a 9 mm gap.
Multi-hyphenated items. Often you can avoid multi-word hyphenated adjectives by rewording, particularly where converted units are involved (the 12-hectare-limit (29.6-acre-limit) rule might be possible as the rule imposing a limit of 12 hectares (29.6 acres)).
Spacing. Normally don't put a space before or after a hyphen, except when it's "hanging" (see above).
Double hyphens. Don't use -- as a substitute for em or en dashes.*
To indicate disjunction, with several applications.
To convey the sense of to or through, particularly in ranges (pp. 211–19, 64–75%, the 1939–45 war, May–November) and where movement is involved (Dublin–Belfast route); but (−3 to 1, not −3–1). Spell out to when the nearby wording demands it: he served from 1939 to 1941, not he served from 1939–1941; similarly, between 1939 and 1941, not between 1939–1941.
To substitute for to or versus (4–3 win in the opening game, male–female ratio).
To substitute for and between independent elements (Canada–US border, blood–brain barrier, time–altitude graph, diode–transistor logic, Lincoln–Douglas debate; but a hyphen is used in Sino-Japanese trade, in which Sino-, being a prefix, lacks lexical independence.) If the elements operate in conjunction, rather than independently, use a hyphen.
To distinguish joint authors from a double-barreled (hyphenated) name: (the Smith–Hardy paper has two authors, but the Jones-Martinez paper has one.
In lists, to separate distinct information within points—for example, between track titles and durations, and between musicians and their instruments, in articles about music albums. In this role, en dashes are always spaced.
As a stylistic alternative to em dashes (see below).
Spacing. All disjunctive en dashes (Category 1, above) are unspaced, except when there is a space within either one or both of the items: the New York – Sydney flight; the New Zealand – South Africa grand final; June 3, 1888 – August 18, 1940, but June–August 1940.
Em dashes (—) indicate interruption in a sentence. They are used in two roles.
Parenthetical (WP—one of the most popular web sites—has the information you need). A pair of em dashes for such interpolations is more arresting than a pair of commas, and less disruptive than parentheses (round brackets).
As a sharp break in the flow of a sentence—sharper than is provided by a colon or a semicolon.
In both roles, em dashes are useful where there are already several commas; em dashes can clarify the structure, sometimes removing ambiguity. Use them sparingly—they are visually striking.
Spacing. Em dashes should not be spaced.
Regular alternative to em dashes. Spaced en dashes – such as here – can be used instead of unspaced em dashes. One style should be used consistently in an article.
No alternatives. Don't use an en dash (–) or hyphen (-) for a negative sign or subtraction operator; instead, use the Unicode character for the minus sign (−, keyed in as −).
Exception. In code, a hyphen may be used.
Spacing. Negative signs (−8 °C) are unspaced; subtraction operators (42 − 4 = 38) are spaced.**
Clearly, they haven't read Wikipedia's Manual of Style.
Hyphens and dashes are basic to stylish writing in English. Even if your readers aren't quite sure of the precise rules that govern their use, their reading will be easier and their comprehension aided by your systematic use of these punctuation marks. The Manual of Style clearly sets out how to use all three punctuation symbols: hyphens (-), en dashes (–) and em dashes (—). If these three symbols are hard to distinguish visually (- – —), you may need to change your font or browser to a standard one that renders them properly.
If you're unsure how to key in en and em dashes, please see this.
Here, we present texts in which hyphens and/or dashes may be either wrongly used or wrongly absent; in other words, some of the examples are wrong, and some are right. Remember, it's mostly a matter of:
whether to use a symbol at all;
if so, whether to use a hyphen or en dash; and in a few cases
whether the symbol should be spaced or unspaced.
Em dashes are a quite separate beast, and much easier to use. Many writers don't use them at all; they use spaced en dashes – like this – instead of unspaced em dashes—like this—for their "interrupters". It's up to you.
The exercises: unfolding design. Each exercise below will present you with a portion of text in which you can correct the (mis)usage of hyphens and dashes. They are designed to be done in your head, without typing. Each one unfolds in stages that you control: first, the problem text, then a hint to help you along; then a solution; and finally an explanation.
The case was upheld by the Supreme Court in a 4–3 judgement.
What word could be substituted for the symbol?
The en dash is correct.
An en dash can stand for "to", "against" or "versus"—these functions are all disjunct (indeed, adversarial) relationships between the items, so a squashy little hyphen (a 4-3 judgement) would be wrong. It's the same for sports scores: "The Vancouver Canucks massacred the New York Islanders in an 11–2 victory" (not "an 11-2 victory").
By mid-afternoon, the low pressure area had moved east into Ontario.
Double adjectives often require hyphenation; and there's another issue. No en dashes this time.
By mid-afternoon, the low-pressure area had moved east into Ontario.
*The presence of the first hyphen is correct, and the absence of a hyphen from the second double adjective is incorrect.
"Mid-" anything is usually hyphenated by convention, probably because "mid" itself isn't a whole word, yet doesn't jam into the noun nicely the way some prefixes do.
"Low-pressure area"—the noun is "area", qualified by a double adjective. There's no such thing as a "pressure area", which could be low or high. Same for "upper-level jet stream"; it's not a "level jet stream" that happens to be the upper one of two.
What about "the area of low pressure over the coast"—shouldn't it be "an area of low-pressure"? Nope. There's no double adjective, so no hyphen should be used: here, "low" is an adjective; "pressure" is a noun.
The introduction of contrasting red and blue–striped balls transformed the game.
Two things wrong: the type of symbol, and the need for it in another place as well. Think of the nominal group (that is, the compound noun): "contrasting red and blue-striped balls".
The introduction of contrasting red- and blue-striped balls transformed the game.
There were two problems:
Hyphen, not en dash. An en dash was being used to conjoin two adjectives; this job is reserved for hyphens.
Hanging hyphen. Not just "blue", but "red" requires a hyphen to link it to the coupled word "striped"; there are two double adjectives (qualifying "balls"). The hanging hyphen added to "red" is useful for making this double compound expression neater. It's a rare case in which a hyphen is followed by a space. See MoS.
The 16-kilometre (10-mi) distance between the centres has been a major impediment to economic development.
There's nothing wrong with the choice of units or the fact of the conversion per se.
The example is incorrect: there should be no hyphen where a unit abbreviation is used.
The 16-kilometre (10 mi) distance between the centres has been a major impediment to economic development.
Sure, both main unit and conversion are used as double adjectives, describing "distance". But the ISO rules, which WP agrees with in this case, are that a hyphen must be used to connect a value and a fully named unit ("30-kilogram weights"), but must not connect a value and an abbreviated unit ("30 kg weights"). Fussy, isn't it.
Yep, an en dash is required, since a relationship is indicated where "and" could be substituted. But it's all in the spacing.
The example is incorrect. The Australia – New Zealand Free Trade Agreement.
If either of the items has an internal space (here, "New Zealand") the en dash needs to be spaced as well. This is particularly important in full date ranges, which often appear at the start of Wikipedia articles: "August 3, 1979 – November 30, 1980", not "August 3, 1979–November 30, 1980", ouch.
The season was fully-subscribed months before the first game.
What is the hyphen trying to do? Is "fully", as an adverb, always going to be clearly and easily connected with a subsequent verb? (Yes.)
The hyphen is incorrect: "The season was fully subscribed months before the first game."
Please memorise: "No hyphen after "-ly".
Why? you might ask. Because an "-ly" word, as an adverb, always qualifies a verb (an action), so your readers will already be expecting it to be joined grammatically to the next word. It's not rocket science. This is one of the most common hyphen glitches on Wikipedia.
The Franco–Prussian War (19 July 1870—10 May 1871) resulted in a complete victory for the Prussians and foreshadowed the unification of Germany.
Both dashes are wrong; why?
The Franco-Prussian War (19 July 1870 – 10 May 1871) resulted in a complete victory for the Prussians and foreshadowed the unification of Germany.
*Word particle. A hyphen should be used in favour of an en dash between two items if one item (or both) is just a word particle; "Franco-" is such a particle – a mere prefix – whereas "French" would be the full-word equivalent ("the French–Prussian War", with an en dash; but by convention this is not the expression). Same for "Sino-Russian War", as opposed to "Chinese–Russian trade agreement". An acronym or abbreviation in this context would count as a full word ("the recent UK–Iceland spat over financial matters").
Range. The em dash in the date was wrong: ranges require an en dash, and if either item has an internal space, as in both dates here, the en dash must also be spaced, on both sides. Many editors use a non-breaking space on the left side of the en dash when it's spaced ("... 74211225324956/55483814021916/3249483277/0125///////////// ...").
At -25°C, it was unseasonably warm in Yellowknife.
There are two things wrong.
At −25 °C, it was unseasonably warm in Yellowknife.
*Minus sign. First, you may think this is an en dash, but it's a minus sign, which mathematicians are fussy about. Until early 2008, MOS used to allow en dashes for negative signs (−3.5) and subtraction operators (4 − 1 = 3); not any more. A minus sign is very subtly different from an en dash—something to do with lying higher on the line. Squint and ... you won't see it. Here they are: hyphen, minus sign, the en dash, and the em dash: - − – — . As a superior race, these mathematicians have the run of the place. Hmmmph.
Value–unit space. Second, MOS insists on a space between all values (−25) and units (°C), with two minor exceptions (angular degrees and geographical coordinates); so it's −25 °C, not −25°C. There's no space between the minus sign and the value (never − 25)
Keying it in. Take a look at how you should key it in: "At −25 °C, it was unseasonably warm in Yellowknife."