This is “Knowing When to Use Hyphens”, section 18.9 from the book Writers' Handbook (v. 1.0).
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Some hyphen usage rules are set requirements, such as in certain compound words and fractions and numbers. Other hyphen usage rules are subjective or situation-specific, such as with certain compound words, prefixes, confusing situations, and continuations to the next line of text.
Some standing compound words are written with hyphens, some as one word without a hyphen, and some as two words without a hyphen.
Examples of compound words that are written with hyphens: merry-go-round, over-the-counter, six-year-old, son-in-law
Examples of compound words that are written as one word with no hyphen: drywall, firefly, softball, toothpaste
Examples of compound words that are written as two separate words without a hyphen: high school, middle class, peanut butter, post office
Other rules for hyphens in compound words include the following:
Fractions and numbers are actually compound words and as such, could be included in Section 18.9.1 "Using Hyphens with Compound Words". But just to be clear, let’s review them briefly here.
Use hyphens to write all two-word numbers between twenty-one and ninety-nine. Also, use hyphens when writing those numbers within larger numbers. Hyphenate a fraction you are expressing as a single quantity, regardless of whether you are using it as a noun or as an adjective.
Use hyphens in certain situations to add prefixes and suffixes to words.
To join a capitalized word to a prefix
To join a number to a prefix
To join a single capital letter to a word
To join the prefixes all-, ex-, quasi-, and self- to words
To join the suffixes -elect, -odd, and -something to words
Sometimes a hyphen can separate two visually alike words from each other. Consider that the use of the hyphen in the first of the following two sentences helps to avoid confusion that would be generated without the hyphen.
Try these exercises without using any words that were given as examples in this section.