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There are no spaces before or after a hyphen. Hyphens can be used to combine words, provide clarity, or split words.
1. To break a word at the end of the line (Always split between syllables.)
2. Numbers: hyphenate numbers twenty-one through ninety-nine
3. Fractions: hyphenate in noun, adjective, and adverb forms except when the second element is already hyphenated (Exception: when discussing the individual parts, do not hyphenate—We cut the pie into four quarters.)
4. Compound modifiers that modify a noun: two or more words that together express a single idea are hyphenated if they precede a noun (e.g., brown-eyed girl, much-needed rest).
Exceptions to Number 4
1. If the adverb in an adverb/adjective or participle compound ends in -ly don’t hyphenate.
2. Don’t hyphenate after very, most, least, more, less
3. If the compound modifier is universally known and understood and/or there is no possible confusion of misreading, don’t hyphenate (e.g., chocolate chip cookie).
4. Compound modifiers formed completely of capitalized words should not be hyphenated (e.g., African American child).
5. Compounds with specific terms (Reference: The Chicago Manual of Style)
ache: always closed (e.g., toothache)
all: adjectival phrases hyphenated before or after a noun; adverbial phrases open (e.g., all-out war, all along)
book: open unless in the dictionary (e.g., reference book, cookbook)
borne: normally closed but hyphenated after words ending in b and after words of three or more syllables (e.g., waterborne, mosquito-borne)
cross: noun, adjective, and adverb forms hyphenated, except some permanent compounds (e.g., cross-reference, crossbow)
e: hyphenate (e.g., e-book, e-mail)
elect: usually hyphenated unless the name of the office consists of two or more words. (e.g., president-elect)
ever: hyphenate before a noun, except some permanent compounds (e.g., ever-ready, everlasting)
ex: hyphenate (e.g., ex-boyfriend)
fold: closed unless formed with a hyphenated number (e.g., tenfold, twenty-five-fold)
6. Normally open compound nouns may sometimes be hyphenated for clarity if preceded by a modifying adjective
Example: house cat vs. gray house-cat (add hyphen to make clear gray is describing the cat not the house)
Example 2: wine cellar vs. underground wine-cellar
7. Hanging hyphen: when the second part of the hyphenated expression is omitted, the hyphen is retained
Example 1: five- to ten-minute intervals
Example 2: fifteen- and sixteen-year-old students
Example 3: both over- and underestimated costs