Before we get started, let's consider the three options for combining words (or parts of words); I'll be using these categories throughout the post:
Open: Parts separated by a space (e.g., "attorney general" or "running shoes")
Hyphenated: Parts connected with a hyphen (e.g., "father-in-law" or "self-reliance")
Closed: Parts written as one word (e.g., "crossbow" or "hypertension").
1. Most Importantly, Always Check The Dictionary
This rule is listed first because it should be your first thought when dealing with any prefix or compound. If the word or phrase is permanent (i.e., listed in the dictionary), use the form that your dictionary provides. Be mindful of the listed part of speech, though; many compounds are open as nouns but hyphenated as adjectives, for instance. If a word or phrase is temporary (i.e., not in the dictionary), follow the tips below.
Use whichever dictionary suits your purposes best; if you are unsure, I recommend Merriam-Webster for U.S. English and Oxford for U.K. English. Note that these dictionaries differ greatly in their treatment of compounds. Whereas Merriam-Webster tends more toward closed forms, Oxford lists more open and hyphenated forms.* Also be sure to follow any rules listed in your style guide (if you are using one); in some cases, a style guide's advice will differ from that of its recommended dictionary.
* Be careful, though: Oxford lists many forms with closed prefixes, and in a couple cases, it lists closed forms (e.g., "email" and "healthcare") for terms that M-W is still waffling on (it lists both "email" and "e-mail" and both "health care" and "healthcare").
Examples: Merriam-Webster has "northeast," "socioeconomic," "twentysomething," and "preexist." Oxford has "north-east," "socio-economic," "twenty-something," and "pre-exist."
2. For Prefixes, Default To Closed
Because prefixes are part of a word and do not (generally) stand on their own, they can be closed up next to their words in most cases. (See #3 and #4 below for exceptions.) Note that U.S. English is more likely to close up prefixes than U.K. English is, but in both cases, the hyphens after prefixes tend to be dropped as terms become more common. If a term is clear without the hyphen, you can probably omit it.
Examples: "antibiotic," "cisgender," "cooperative," "hyperactive," "intersectional," "microeconomics," "midlevel," "nonnegotiable," "overzealous," "postwar," "premodern," "reedit," "subzero," and "undiminished"
3. Hyphenate Prefixes Before A Numeral Or Capital Letter
Two of the exceptions to tip #2 involve cases in which it looks weird to close up a prefix with what follows it: Use a hyphen after a prefix if the root starts with a numeral or a capital letter. For example, "mid50s" and "preColumbian" look weird; you'd instead say "mid-50s" and "pre-Columbian."
Examples: "anti-Russian," "pro-Kennedy," "neo-Nazi," "pre-2000," and "sub-4.0"
4. Hyphenate After Prefixes When Needed To Prevent Misreading
The other exceptions involve clarity. If there is any chance that a word with a prefix could be misread, use a hyphen to ensure that the meaning is clear.
For instance, add hyphens when the entire prefix is repeated in the root (e.g., "un-unionized" and "re-regulate"). In addition, always hyphenate a prefix that ends in "i" or "a" if the root also starts with that letter (e.g., "anti-inflammatory" and "mega-architecture"). For prefixes resulting in a double "e" or "o," some words are closed (e.g., "preempt" and "coordinate"), but others use hyphens (e.g., "pre-engineered" and "co-opt"). Check the dictionary first, and if a term is not listed, use a hyphen to be safe.
Finally, always hyphenate when the root is itself a compound. This helps the reader parse the full phrase, as in "mid-twentieth-century leaders" and "step-granddaughter."
Examples: "anti-intellectual," "co-conduct," "non-beer drinker," "intra-arterial," "semi-unsuccessful," and "re-cover" (vs. "recover")
5. For Multiword Compounds Used As Nouns, Default To Open
The rest of the tips deal with compounds comprising multiple words (rather than a prefix/suffix and a root). For this tip, if such a compound acts as a noun, you will usually use the open form. However, as stated in tip #1, always check the dictionary; some permanent compounds are closed or hyphenated, even as nouns (e.g., "schoolhouse" and "decision-making"). If a noun phrase is not in the dictionary, however, it is usually best to use the open form.
Examples: "farm owner," "tenure track," "reference book," "dance shoes," "sixth floor," and "headline style"
6. For Multiword Modifiers Before A Noun, Default To Hyphenated
If a multiword compound is used as a modifier before a noun, however, you will generally hyphenate it for clarity (but if the noun form is closed, the modifier is also closed, as in "schoolhouse repairs").
Note that, if a multiword term is very common in a given field, it can be left open when the audience consists of members of that field (as confusion is unlikely in that case). For instance, "red blood cell count" does not need hyphens in a medical context, but it might need to be hyphenated as "red-blood-cell count" for general audiences, who might not be as familiar with the term.
Examples: Contrasting with #5, "farm-owners association," "tenure-track professor," "reference-book collection," "dance-shoe store," "sixth-floor office," and "headline-style capitalization"; contrasting with #7, "toll-free call," "paint-splattered shirt," "cutting-edge technology," "off-the-wall suggestion," and "much-needed rest"
7. For Multiword Modifiers After A Noun, Default To Open
When a modifier comes after its noun, misreading is much less likely, so it is usually fine to omit the hyphen. For instance, though you would hyphenate "a full-term birth," you would not hyphenate "The birth was full term."
Examples: "Each call is toll free," "everyone's shirt was paint splattered," "our technology is cutting edge," "the suggestion proved to be off the wall," and "the rest was much needed."
8. Don't Hyphenate "-ly" Adverbs, "Very," "More"/"Most," Or "Less"/"Least"
Nouns and adjectives can be combined in various ways, so hyphens can help readers determine which meaning the writer intended. However, adverbs that end in "-ly"* typically cannot be misread, as they always modify the following adjective or verb. Thus, you don't need to hyphenate after them, even when they are used in a compound modifier, as in "fully stocked bar" or "widely praised novel."
* Note that some words that end in "-ly" are not adverbs: For instance, "lonely," "costly," "homely," "lively," "manly," "saintly," and "yearly" are adjectives (see others here); "comply" and "reply" are verbs; and "monopoly" and "barfly" are nouns. If a word end in "-ly" but is not an adverb, you'll likely still need to hyphenate it when used in a modifier before a noun, as in "the scholarly-paper archive" or "a family-run business." As always, check the dictionary if you are unsure about a part of speech.
Along the same lines, in most cases, "very," "more"/"most," or "less"/"least" do not need hyphens, as in "a very popular program" or "a less successful method." However, watch out for cases in which a hyphen is needed to prevent ambiguity; for instance, you'd say "the most educated candidates" when referring to the largest number of educated candidates ("most" modifies the noun) but "the most-educated candidates" when referring to the candidates with the most education ("most" modifies the adjective).
Examples: "most efficient method," "poorly constructed box," "partly completed task," "slowly moving car," "very interesting course," and "least hydrated plant"
9. Don't Add Hyphens Within Non-English Phrases Or Distinct Proper Nouns
There are a few other cases in which modifying phrases are not hyphenated because they are clear on their own. For instance, if the modifier is a phrase taken from another language (often Latin), then you do not hyphens, as in "a priori argument" or "ex post facto rationalization." Such non-English-origin phrases are often italicized, but even if they are not, they stand apart and thus don't need hyphens. (Of course, if the original phrase has hyphens, as in "a tête-à-tête meeting," leave them in.)
Similarly, combinations of proper terms that describe a distinct geography, nationality, or ethnicity are not hyphenated as nouns or adjectives: "African Americans," "Middle Eastern leaders," "South Central Europe," etc. Hyphens in such terms would not add any meaning, so you can omit them. However, combinations that refer to multiple places or peoples are hyphenated: "Chinese-Russian relations," "Australia-New Zealand travel," etc. In these terms, the hyphens indicate that there is an interaction between the proper terms, so you can't leave the punctuation out.
Examples: "in vitro fertilization," "roman à clef story," "South Asian American," "East Central Africa," and "Pacific Northwest coast"
10. Hyphenate Equal Terms and Combining Forms
This tip covers combinations in which the two parts are roughly equal in importance. For instance, always hyphenate equally weighted nouns, as in "a writer-director" or "the center-left." However, do not hyphenate if the nouns are not equal in weight (i.e., if one modifies the other), as in "the wood furniture" or "an energy bar." Here's a handy trick: If you can swap the order of the nouns without changing the meaning, use a hyphen; if you can't, leave the hyphen out.
In addition, new terms are often coined by blending two equally weighted terms using a "combining form" of the first term. For instance, "physico" is the combining form of "physical," and it can be used to create words such as "physico-social" or "physico-emotional." In such cases, check the dictionary first and use the hyphenated form if the term isn't listed (the dictionary-listed forms are usually closed).
Examples: "city-state," "parent-coach," "electro-visual stimulation," "chemico-sensory measurements"
I haven't covered every situation in this post (in particular, the rules for hyphens with numbers are complicated, so I left them off). However, if you follow these tips (especially #1), you'll have no trouble getting your hyphens right in most situations.
Do you have any questions about hyphens? Do you need advice about a particular phrase? If so, please leave a comment or email me at firstname.lastname@example.org. I'd be happy to help. And, of course, if you prefer to have me take care of all the punctuation (and more) in your document, I'd encourage you to request a free, no-obligation quote at www.elevationediting.com. Satisfaction guaranteed on all edits!
Thanks for reading, as always.