The hyphen is a connector; it may join “what is similar and also what is disjunctive….it divides as well as marries.”2 The hyphen connects words, prefixes, and suffixes permanently or temporarily. Certain compound words always contain hyphens. Such hyphens are called orthographic. Examples are merry-go-round, free-for-all, and mother-in-law. For temporary connections, hyphens help prevent ambiguity, clarify meaning, and indicate word breaks at the end of a line.
In general, when not otherwise specified, hyphens should be used only as an aid to the reader’s understanding, primarily to avoid ambiguity. For capitalization of hyphenated compounds in titles, subtitles, subheads, and table heads, see 10.2.2, Capitalization, Titles and Headings, Hyphenated Compounds.
Hyphenate temporary compounds according to current dictionary usage and the following rules:
Hyphenate a compound that contains a noun or an adverb (except for adverbs ending in -ly; see below, When Not to Use Hyphens) and a participle that together serve as an adjective modifying the noun they precede. Do not use the hyphen if the compound follows the noun.
decision-making methods (But: methods of decision making)
most-read work in the collection (But: The work was the most read in the collection.)
It was a placebo-controlled trial. (But: The trial was placebo controlled.)
This is a well-edited volume. (But: This volume is well edited.)
The rash was a treatment-related adverse event. (But: The adverse event was treatment related.)
Hyphenate a compound adjectival phrase when it precedes the noun it modifies but not when it follows the noun.
side-by-side placement (But: placed side by side)
Hyphenate an adjective-noun compound when it precedes and modifies another noun but not when it follows the noun.
low-quality suture material (But: suture material of low quality)
highest-quality printing (But: printing of highest quality)
low-density resolution (But: resolution of low density)
low-density nerve fibers (But: nerve fibers of low density)
high-altitude sickness (But: sickness at high altitude)
very low-birth-weight children (But: children of very low birth weight)
low-molecular-weight heparin (But: heparin of low molecular weight)
very low-density lipoprotein (But: lipoprotein of very low density)
For compound adjectival phrases, adverb-participle compounds, and adjective-noun compounds that have become commonplace and familiar in everyday usage, hyphenate these phrases or compounds whether they precede or follow the noun they modify. (Follow The Chicago Manual of Style, 15th edition, to verify.)
the commitment was long-term
the vaccinations were up-to-date
equipment that was state-of-the-art
Hyphenate a combination of 2 or more nouns used coordinately as a unit modifier when preceding the noun but not when following.
the Binet-Simon test (But: the test of Binet and Simon)
Beer-Lambert law (But: the law of Beer and Lambert)
Charcot-Marie-Tooth disease (But: the disease described by Charcot, Marie, and Tooth)
Hosmer-Lemeshow goodness-of-fit test (But: the goodness-of-fit test of Hosmer and Lemeshow)
the physician-patient relationship (But: the relationship between the physician and the patient)
Presentation of ratios as numbers or abbreviations is an exception to this rule. In ratios presented as numbers or abbreviations, use a colon (see 8.2.3, Colon). For ratios presented as words, use the word to or, if the word combination has become accepted as a single term, such as cost-benefit analysis, a hyphen.
Hyphenate a combination of 2 nouns of equal participation used as a single noun. (See also 8.4.1, Forward Slash [Virgule, Solidus], Used to Express Equivalence or Duality.)
William Carlos Williams was a physician-poet.
W. Somerset Maugham is considered a great physician-writer.
She is an obstetrician-gynecologist.
Provide the best health care for all, says the citizen-patient; but don't allow costs to rise, says the citizen-taxpayer.
The physician-patient may become impatient with treatment.
The study involved 1000 patient-years.
(But: onlooker, passerby, handout, workup, makeup)
Hyphenate a compound in which a number is the first element and the compound precedes the noun it modifies.
18-factor blood chemistry analysis
ninth-grade reading level
Hyphenate 2 or more adjectives used coordinately or as conflicting terms whether they precede the noun or follow as a predicate adjective.
The false-positive test results were noted.
The test results were false-positive.
We performed a double-blind study.
The test we used was double-blind.
The author provided black-and-white illustrations.
The author’s illustrations were black-and-white.
Hyphenate color terms in which the 2 elements are of equal weight.
blue-black lesions (lesions were blue-black)
(But: bluish gray lesions)
Hyphenate compounds formed with the prefixes all-, self-, and ex- whether they precede or follow the noun.
the patient’s ex-husband
(Note: With the prefix vice, follow the latest edition of Merriam-Webster’s Collegiate Dictionary, eg, vice-chancellor, vice-consul, but vice president, vice admiral.)
Hyphenate compounds made up of the suffixes -type, -elect, and -designate.
Hyphenate most contemporary adjectival cross- compounds (consult the latest edition of Merriam-Webster’s Collegiate Dictionary for absolute accuracy; there are exceptions, eg, crossbred, crosshatched, crossover, crossmatch, cross section).
Most nouns that begin with quasi are not hyphenated but instead are set open (eg, quasi diplomat), although some are closed up (eg, quasicrystal, quasiparticle). Follow the latest edition of Merriam-Webster’s Collegiate Dictionary.
Hyphenate some compounds in which the first element is a possessive. Consult the latest edition of Merriam-Webster’s Collegiate Dictionary.
Hyphenate all prefixes that precede a proper noun, a capitalized word, a number, or an abbreviation.
Note: There is growing recognition and acceptance of the use of a stand-alone prefix with a hyphen when an alternative unhyphenated prefix follows.
We found a need for pre- and postoperative examination.
Patients were categorized as hyper- or hypotensive.
This could be an in- or outpatient procedure.
JAMA and the Archives Journals choose not to follow this trend and instead would use the following:
We found a need for preoperative and postoperative examination.
Patients were categorized as hypertensive or hypotensive.
This could be an inpatient or outpatient procedure.
When 2 or more hyphenated compounds have a common base, omit the base in all but the last. In unhyphenated compounds written as 1 word, repeat the base.
first-, second-, and third-grade students
10- and 15-year-old boys
anterolateral and posterolateral aspects
Hyphenate compound numbers from 21 to 99 (cardinal and ordinal) when written out, as at the beginning of a sentence. (See 19.1, Numbers and Percentages, Use of Numerals.)
Thirty-six patients were examined.
Twenty-fifth through 75th percentile rankings were shown.
One hundred thirty-two people were killed in the plane crash.
A two-thirds majority was needed.
The flask was three-fourths full.
Three-fourths of the questionnaires were returned.
Use hyphens to avoid ambiguity. If a term could be misleading without a hyphen, hyphenate it. As with the use of commas to indicate pauses, the use of the hyphen to provide clarity may be subjective. What is clear to one person may be a source of ambiguity to another. Use the following guidelines and a healthy dose of common sense.
a small-bowel constriction (constriction of the small bowel)
a small bowel constriction (a small constriction of the bowel)
a single-specialty center (a center devoted to a single specialty)
a single specialty center (1 center devoted to a specialty)
a large-bowel resection (resection of the large bowel) (Better: a colon resection)
a large bowel resection (a large resection of the bowel)
a solid-organ transplantation program (a program for transplantation of solid organs)
a solid organ transplantation program (a program for organ transplantation that is solid, ie, well established) (Better: a well-established transplantation program)
Use a hyphen after a prefix when the unhyphenated word would have a different meaning.
Note: Do not hyphenate other forms of these words for which no ambiguity exists: retreatment, recreational.
Occasionally, a hyphen is used after a prefix or before a suffix to avoid an awkward combination of letters, such as 2 of the same vowel or 3 of the same consonant (with exceptions noted below, When Not to Use Hyphens). Follow the latest edition of Merriam-Webster’s Collegiate Dictionary or Dorland's or Stedman's medical dictionary.
(Some exceptions to this rule include microorganism, cooperation, reenter [see below, When Not to Use Hyphens].)
In complex modifying phrases that include suffixes or prefixes, hyphens and en dashes are sometimes used to avoid ambiguity. (See also 8.3.2, Dashes, En Dash.)
non–Q-wave myocardial infarction
Expressing Ranges and Dimensions.
When expressing ranges or dimensions used as modifiers, use hyphens and spacing in accordance with the following examples in the left-hand column. The alternatives in the right-hand column give the expression of dimensions when not used as modifiers.
in a 10- to 14-day period
10 to 14 days' duration
a 3 × 4-cm strip
a strip measuring 3 × 4 cm
a 5- to 10-mg dose
a dose of 5 to 10 mg
in a 5-, 10-, or 15-mg dose
in a dose of 5, 10, or 15 mg
a 3-cm-diameter tube
a tube 3 cm in diameter
a lesion 5 mm thick
In the text, do not use hyphens to express ranges. (See 19.4, Numbers and Percentages, Use of Digit Spans and Hyphens.)
The adverse events were experienced by 5% to 10% of the group.
The exceptions to this rule about ranges are for (1) ranges expressing fiscal years, academic years, life spans, or study spans and (2) ranges given in parentheses.
We present results from the 2002-2004 Renal Study Group.
The patients' median age was 56 years (range, 31-92 years).
Note that no hyphens are needed in the following cases:
a 3 to 4 ratio
a case of mild to moderate pruritus
Use hyphens to indicate division of a word at the end of a line (follow the latest edition of Merriam-Webster’s Collegiate Dictionary or Stedman's or Dorland's medical dictionary).
When Not to Use Hyphens.
Rules also exist for when not to use hyphens.
The following common prefixes are not joined by hyphens except when they precede a proper noun, a capitalized word, or an abbreviation: ante-, anti-, bi-, co-, contra-, counter-, de-, extra-, infra-, inter-, intra-, micro-, mid-, multi-, non-, over-, pre-, post-, pro-, pseudo-, re-, semi-, sub-, super-, supra-, trans-, tri-, ultra-, un-, under-.
Retain the hyphen when the term after the prefixes anti-, neo-, pre-, post-, and mid- is a proper noun or a number (see also above, Temporary Compounds), eg, mid-1900s, mid-Atlantic crossing.
The following suffixes are joined without a hyphen, with exceptions if the clarity would be obscured (see Temporary Compounds above): -hood, -less, -like, -wise.
Some combinations of words are commonly read together as a unit. As such combinations come into common use, the hyphen tends to be omitted without a sacrifice of clarity. Use the latest editions of Merriam-Webster’s Collegiate Dictionary and Dorland's and Stedman's medical dictionaries as guides to common usage (eg, broad-spectrum antibiotics is hyphenated in Dorland's; open heart surgery, deep venous thrombosis, and small cell carcinoma are not). For terms not found in these sources, use a reader’s perspective and the context as guides (eg, JAMA and the Archives Journals hyphenate soft-tissue, as in soft-tissue mass, to avoid confusion). When no confusion is likely, leave open. If there is a possibility of confusion, hyphenate. A short list of examples that can usually be presented without hyphens is given below.
amino acid levels
medical school students
birth control methods
natural killer cell
bone marrow biopsy
open heart surgery
deep venous thrombosis
peer review process
foreign body infiltrate
primary care physician
health care system
public health official
inner ear disorder
small cell carcinoma
lower extremity amputation
tertiary care center
Do not hyphenate names of disease entities used as modifiers.
basal cell carcinoma
connective tissue tumor
hyaline membrane disease
sickle cell trait
clam diggers' itch
grand mal seizures
Do not use a hyphen after an adverb that ends in -ly even when used in a compound modifier preceding the word modified; in these cases, ambiguity is unlikely and the hyphen can be dispensed with.
the clearly stated purpose
biologically mediated therapy
a highly developed species
previously published recommendations
clinically derived databases
clinically relevant variables
Do not hyphenate names of chemical compounds used as adjectives.
sodium chloride solution
tannic acid test
Most combinations of proper adjectives derived from geographic entities are not hyphenated when used as noun or adjective formations.
Pacific Rim countries
Southeast Asian countries
Central American customs
(But: Scotch-Irish ancestry. Here the hyphen is used to indicate 2 countries of origin.)
Do not hyphenate Latin expressions or non–English-language phrases used in an adjectival sense. Most of these are treated as separate words; a few are joined without a hyphen. Follow the latest edition of Merriam-Webster’s Collegiate Dictionary.
an a priori argument
an ex officio member
per diem employees
prima facie evidence
in vivo specimens
carcinoma in situ
café au lait spots
post hoc testing
Note that when post is used as a combining adjectival form, as in postmortem examination, it is set closed up. When it is used as an adverb, as in post hoc testing, it is set as 2 separate words. This distinction is apparent in the examples below:
depression occurring post partum
Do not hyphenate modifiers in which a letter or number is the second element.
grade A eggs
study 1 protocol
type 1 diabetes mellitus
Compound Official Titles.
Hyphenate combination positions of office but not compound designations as follows:
(But: past vice president, executive vice president, past president)
Special combinations may or may not necessitate the use of hyphens. Consult Stedman's, Dorland's, and the latest edition of Merriam-Webster’s Collegiate Dictionary. (See 15.0, Nomenclature, and 17.0, Greek Letters.)
I beam (I-shaped beam)