Hyphens can also eliminate ambiguity. For example, the hyphen in much-needed clothing shows that the clothing is greatly needed rather than abundant and needed. Where no ambiguity could result, as in public welfare administration or graduate student housing, hyphenation is unnecessary. (Chicago Manual 7.80)

Compound Modifiers

Hyphens are typically used in compound modifiers that precede a noun: a late-day rally, a full-time job, a come-what-may attitude. When such compounds follow the noun they modify, hyphenation is usually unnecessary.

Hyphens are not used when the compound modifier preceding a noun:

  • includes the word very or an adverb ending in –ly: a very strong will, a dangerously swollen stream
  • when the meaning is clear without hyphenation: sales tax bill; foreign aid plan
  • is an adjective relating to geography or nationality, unless the first term is a prefix or “between” is implied: Middle Eastern countries, the North Central region, African American literature; but the Franco-Prussian War, Sino-Tibetan languages, Afro-Atlantic contemporary art, the U.S.-Canada border (between is implied in the last example)
  • is a number and percentage: a 10 percent raise
  • is a chemical term: the sodium chloride solution
  • is a foreign phrase, unless hyphens appear in the original language: an a priori argument; in vitro fertilization; a tête-à-tête approach

Compound modifiers containing the word more, most, less, least, or very are also usually open (no hyphen) unless one is needed to avoid ambiguity: a more thorough exam; the most efficient method; a less prolific artist; the least understood approach; a very much needed addition; but: the lesser-paid colleague.

Words Formed with Prefixes

Words formed with prefixes are normally closed (no hyphen). For example:

antebellum, antihero, bisexual , bioecology, coequal, counterculture, cyberspace, extramural, hypertext, infrastructure, interfaith, intramural, macroeconomics, megavitamin, metaethical, microeconomics, midcentury, minicourse, multimedia, neonatal, noncredit, overconscientious, postbaccalaureate, premodern, prerequisite, protolanguage, pseudomodern, reunify, semiprivate, subzero, superannuated, transcontinental, ultraorganized, unfunded, underemployed

However, a hyphen should appear:

  • before a capitalized word or numeral: sub-Saharan, pre-1950, pro-Canadian
  • before a hyphenated compound: non-tenure-track faculty (before an open compound, an en dash is used: post–World War II)
  • to separate two i’s, two a’s, two e’s, and other combinations of letters or syllables that might be misleading: anti-intellectual, extra-alkaline; coordinate, but co-op (a cooperative, to distinguish from coop, a cage for animals), co-opt
  • after pro- when the compound word means “in favor of”
  • to separate the repeated terms in a double prefix: sub-subentry
  • when a prefix or combining form stands alone: over- and underused, macro- and microeconomics
  • when using the prefix co- to form nouns, adjectives, or verbs that indicate occupation or status: co-author, co-chair, co-defendant, co-host, co-owner, co-pilot, co-signer, co-sponsor, co-star, co-worker (this is AP style and an exception to the Chicago Manual and, in some cases, Merriam-Webster)