Hyphens and compound words
Hyphens show which words belong together and can distinguish meanings that would otherwise be confused. A compound's spelling also depends on its grammatical role and established usage: related forms may be separate words, hyphenated or written as one word.
Compound modifiers
A hyphen links words that jointly qualify a noun, helping readers decode their relationship without interrupting the argument. Examples include treatment-related adverse events, high-quality materials, low-density resolution and placebo-controlled trial. A large-vein injury identifies the type of vein affected, while removing the hyphen changes how the phrase can be read. In serotonin-stimulated serotonin release, the hyphen makes serotonin-stimulated a modifier of serotonin release; without it, stimulated can be read as a verb.
A compound must be marked fully enough to show the intended links. Two colour graphics terminals becomes two-colour-graphics terminals, and heavy metal-containing pigments becomes heavy-metal-containing pigments. Other examples are sulphur-containing gases, a waterproof waste-disposal bag and all-upper-case words.
The grammatical role can determine whether a hyphen is appropriate: the steak is well done contrasts with a well-done steak, readers are well informed with a well-informed readership, and the medication was well tolerated with a well-tolerated medication. Similarly, detail oriented can describe how someone is, while a detail-oriented person uses a compound modifier. A hyphenated word such as well-being retains its established spelling.
A compound adjective such as standard-of-care takes hyphens in standard-of-care treatment, whereas the noun phrase stands separately in the standard of care at this medical center. Number modifiers include a 20-year-old patient, a 2-year study, a 5-min assessment and a 7-day experiment.
An adverb ending in -ly is not joined by a hyphen to the adjective it modifies: markedly enlarged liver, appropriately sized blade and smoothly flowing sentence. An appropriate-sized blade is another accepted form. In extremely low-birth-weight infants, hyphens group low, birth and weight into one adjectival unit; extremely remains separate. This grouping can help non-experts especially, because the unmarked stack can distract them more than it distracts an expert. A number that is part of a name is a different case: type 2 diabetes is not hyphenated.
A suspended hyphen avoids repeating a shared component: a double-layered or treble-layered coating becomes a double- or treble-layered coating. It is acceptable but should be used sparingly because the resulting word cluster can be awkward to process.
Prefixes and distinctions in meaning
A hyphen can distinguish re-treat from retreat, re-sign from resign, and re-form from reform. Prefixes also appear in established solid forms such as pretreatment, nonadjustable and rehabilitated. The prefix non rarely takes a hyphen, but does before a capitalised element, as in non-American.
Open, hyphenated and solid compounds
English compounds can develop from two words through a hyphenated form to one word, as illustrated by high way, high-way and highway. An earlier response to adjacent vowels was a hyphen or dieresis, as in co-operation; solid forms include cooperation and reelect. Freelance is one word and can function as a noun, verb, adverb or adjective.
The open and solid forms can have different meanings: onto expresses movement in he jumped onto the steel girder. Meaning and usage matter when deciding whether adjacent words form one word.
Spelling preferences also differ across styles and dates. In 2007, the tenth edition of the AMA Manual of Style used e-mail throughout, and the New York Times Manual of Style and Usage prescribed that hyphenated form. A 2010 usage preference favoured health care and Web site as separate words, on the grounds that separation made their meaning clearer.
Verb phrases and the nouns formed from them
A verb used with its particle is not hyphenated: follow up patients, the study was set up and the system is backed up every 24 hours. Hyphenation or solid spelling becomes a question when the pair forms a noun or adjective, as in follow-up investigations, there was no follow-up and a build-up of gas. Setup, workup, backup, layout and breakdown illustrate compounds written as one word; choices such as work-up and workup can depend on style and usage.
The practical recommendation is consistency for each term within a document: workup should not alternate with work-up, but different terms need not all follow one pattern. Frequency of use may influence whether a compound becomes established as a solid word, but the point of transition is difficult to predict.
Compounds ending in wide and long
For worldwide, three spelling approaches illustrate the choice between an open, hyphenated and solid form in phrases such as the worldwide incidence of or an increase of 23% worldwide.
- World wide, with a space and no hyphen, is one approach that the usage advice rejects.
- World-wide uses a hyphen to show that wide is linked to world.
- Worldwide is the solid form, recommended as the simplest option requiring the least checking.
The same advice accepts the hyphenated or solid approach for countrywide and companywide and recommends consistency within a text. Hospital-wide and nation-wide, with or without the hyphen, are also in common use.
One style preference favours hyphens in month-long and year-long but recommends avoiding those constructions in medical and scientific prose by stating the duration: a 1-month course or a 1-year sabbatical. That preference retains life-long disability because the idea is difficult to express as economically in another form.
Nouns used as modifiers
Nouns can modify nouns: a mucus plug means a plug of mucus, and phosphorus depletion means depletion of phosphorus. Established attributive combinations can use singular or plural forms, as in toy store and ladies room. Usage fixes particular combinations: arms race and Antiques Roadshow cannot simply exchange their modifying words for arm and Antique. One recommendation is to use a singular modifier in a new term unless there is a strong reason for a plural, while retaining established combinations.
Nonbreaking hyphens
A nonbreaking hyphen keeps a hyphenated expression together on one line. It should be inserted during manuscript preparation, because later layout changes may move an expression to a line boundary and adding the marks afterwards is tedious.