To hyphenate or not?
Compound modifiers are groups of two or more words that jointly modify the meaning of another word. When a compound modifier other than an adverb-adjective combination appears before a term, the compound modifier is often hyphenated to prevent misunderstanding, such as in American-football player or little-celebrated paintings. Without the hyphen, there is potential confusion about whether the writer means a "player of American football" or an "American player of football" and whether the writer means "celebrated paintings" that are little. Compound modifiers can extend to three or more words, as in ice-cream-flavored candy, and can be adverbial as well as adjectival (spine-tinglingly frightening). However, if the compound is a familiar one, it is usually unhyphenated. For example, at least one style guide prefers the construction high school students, to high-school students. Although the expression is technically ambiguous ("students of a high school" / "school students that are on drugs"), it would normally be formulated differently if the latter meaning were intended. Noun–noun compound modifiers are also written (according to some) without a hyphen when no confusion is likely: grade point average and department store manager. When a compound adjective follows the term it applies to, a hyphen is typically not used. For example, "that gentleman is well respected", not "that gentleman is well-respected". Some authorities differ, and recommend the hyphen when the compound adjective follows the verb to be or any of its inflections. According to most style guides, hyphens should not be used in adverb-adjective modifiers when the adverb ends in -ly. For example, wholly owned subsidiary and quickly moving vehicle because the adverbs clearly modify the adjectives and "quickly" cannot modify "vehicle". However, if an adverb can also function as an adjective, then a hyphen may be or should be used for clarity, depending on the style guide. For example, the phrase more-important reasons ("reasons that are more important") is distinguished from more important reasons ("additional important reasons"), where more is an adjective. (In contrast, the hyphen in "a more-important reason" is not necessary to distinguish it from "a more important reason" since both mean the same thing.) A mass-noun example is the following: more-beautiful scenery as distinct from more beautiful scenery. Strunk & White notably does not have a prohibition against hyphen use in compound adjectives ending in -ly. Rather, as with other hyphen usage, the key is "common sense" and writing for clarity. See id. Hyphens are used to connect numbers and words in forming adjectival phrases (particularly with weights and measures), whether using numerals or words for the numbers, as in 28-year-old woman and twenty-eight-year-old woman or 320-foot wingspan. The same usually holds for abbreviated time units. Hyphens are also used in spelled-out fractions as adjectives (but not as nouns), such as two-thirds majority and one-eighth portion. However, with symbols for SI units—as opposed to the names of these units—both the International Bureau of Weights and Measures and the U.S. National Institute of Standards and Technology recommend use without a hyphen: a 25 kg sphere. When the units are spelled out, this recommendation does not apply: a 25-kilogram sphere, a roll of 35-millimeter film. In English, an en dash ( – ) sometimes replaces the hyphen in hyphenated compounds if either of its constituent parts is already hyphenated or contains a space, for example San Francisco–area residents or public-school–private-school rivalries. En dashes are more proper than hyphens in ranges (pp. 312–14), relationships (blood–brain barrier) and to convey the sense of "to", as in Boston–Washington race. Via Wikipedia