Certain words and phrases tend to evolve from separation to linkage. The trend in English is for frequently used word combinations to "grow together" from two words to one, sometimes passing through a hyphenated stage. The two-word phrase data base, for example, is now most commonly written as one word: database. (It apparently skipped the hyphenated transition phase.)
The best rule to follow for particular words and phrases is to check a recent dictionary. However, the following principles are useful to know.
n Two or more adjectives before a noun that act as one idea (one-thought adjectives) are connected with a hyphen.
Examples: This is a low-budget job. [The sense is not: this is a low job and a budget job. The words low and budget are linked into the single concept of "low-budget."]
First-class decisions require clear-headed thinking.
He has a devil-may-care attitude.
NOTES, VARIATIONS, AND APPLICATIONS
n When the adjectives before the noun act separately, we are usually implying the word and (which we replace with a comma), as in "a ripe, red tomato." The meaning is a ripe tomato and a red tomato or a ripe and red tomato. But a low-budget job, in contrast, is not a low job and a budget job or a low and budget job.
n When the modifying words are positioned differently in the sentence - say, after the noun - the hyphen is usually not used. For instance, well-known has no hyphen in the sentence, "This institution is well known." Note, however, that some two-word expressions are always linked by a hyphen, regardless of position in the sentence; examples are part-time and full-time.
n When we refer to a twelve-year-old boy, the hyphens follow the rule for one-thought adjectives. No hyphens are used when the phrase is positioned differently (i.e., after boy): "The boy is twelve years old." The hyphens are used, though, when we make the phrase a noun, e.g., "He is a typical twelve-year-old." (In a sense, the word boy or child is understood.)
n Use a hyphen in expressions where words have become linked by usage to express one idea.
We can deduce some of these by logic - e.g., mother-in-law, top-notch, fine-tune, X-ray. However, since the language is not always logical, often inconsistent, and rapidly changing with regard to hyphenated words, check an up-to-date dictionary.
n Do not use a hyphen after an adverb ending with -ly.
Examples: a carefully planned project, a recently discovered virus, a fully automated system. (Note that the -ly adverbs are moveable or switchable: e.g., a project planned carefully, a virus discovered recently, a system that is automated fully.)
n Do not use a hyphen in a compound using a comparative or superlative adjective.
Examples: a better built house, a more likely outcome, the best laid plans.
n Do not use a hyphen in chemical terms.
Examples: calcium carbide solution, a hydrogen peroxide mixture.
n Do not use a hyphen in a modifier using a letter or numeral as the second element.
Examples: a Type IV antibody, a Class A priority.
n When written as words, fractions and cardinal numbers consisting of two words are hyphenated.
Examples: twenty-three, twenty-fifth, one-fourth, two-thirds.
n Hyphenate words prefixed by ex-, self-, or all-, and some words prefixed by cross-.
Examples: ex-wife; self-evident; all-inclusive; cross-reference.
NOTE: Cross section (the noun) is two words, but cross-sectional (the adjective) is hyphenated. Moreover, many words prefixed by cross have become solid, e.g., crossword, crossroad.
n Do not hyphenate words prefixed by non, un, in, dis, co, anti, hyper, pre, re, post, out, bi, counter, de, semi, mis, mega, micro, inter, over, and under (among others).
Examples: nonaffiliated, nonemergency, uninfected, inpatient, disorder, disbar, coworker, copayment, antismoking, antimanagement, hyperactive, hyperrealism, preoperative, prejudge, reoccur, readjust, resubmit, postoperative, posttraumatic, outpatient, outmoded, bimonthly, biannual, counterrevolutionary, counterculture, decompress, semifinal, semiannual, misinformed, misprint, megabyte, microcircuit, interconnected, interoffice, overemphasize, override, underrepresent, underestimated.
EXCEPTIONS: When the second element is capitalized, as in Un-American and non-English, a hyphen is used. Also, occasional exceptions exist where the prefix and the second element have not (yet) "grown together," such as de-emphasize, pre-owned, co-op (to distinguish from coop) and anti-inflammatory (and all words with anti- prefix and second element beginning with i),
NOTE: When a prefixed word does not appear in the regular listings in the dictionary, go back to the listing for the prefix itself; many dictionaries give a supplementary list of words here, without definitions. For example, such a list may appear on or near the entry for "non-."
n Do not (usually) hyphenate verb and preposition combinations.
The verb form is not the same as the adjective or noun. We check out (two words - verb) at the checkout (one word - noun). We also call back, send out, phone in, look out, etc. (no hyphens).
n Use hyphens when needed for clarity.
The hyphen is needed, for instance, to distinguish re-sign from resign or re-creation from recreation. It helps to differentiate a dirty-movie theater from a dirty movie-theater.