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18.9 Knowing When to Use Hyphens

Learning Objectives

  1. Recognize compound words that require hyphens all the time and those that require hyphens due to specific situations.
  2. Learn how to use hyphens in writing numbers.
  3. Learn which prefixes and suffixes require the use of a hyphen.

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.

Using Hyphens with Compound Words

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:

  • Hyphenate compound words when they are used together to modify the same word (e.g., “Scout was a quick-witted child”).
  • Do not turn words into a hyphenated compound adjective if words are placed after the word they modify (e.g., “Scout was a child who was quick witted”).
  • Do not hyphenate -ly adverbs and adjectives (e.g., “Georgie has a highly coveted first-run copy,” not “Georgie has a highly-coveted first-run copy”).

Using Hyphens to Write Fractions and Numbers

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.


  • twenty-one
  • four hundred twenty-one
  • two-thirds of the pie
  • a one-quarter share of the profits

Using Hyphens with Prefixes and Suffixes

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



Using Hyphens to Avoid Confusion

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.

  • I think the assistant prosecutor should re-sign.
  • I think the assistant prosecutor should resign.

Key Takeaways

  • Some compound words are always in compound form and some are hyphenated. Writers create other hyphenated compound words for situational needs when two or more words modify the same word and are placed before that word in a sentence.
  • Hyphens are used to separate the words in the numbers twenty-one through ninety-nine and in fractions.
  • You should use a hyphen when adding prefixes to proper nouns or numbers. Also, use a hyphen to join a capital letter to a word and to join certain prefixes (all-, ex-, quasi-, and self-) and suffixes (-elect, -odd, and -something).


  1. Try these exercises without using any words that were given as examples in this section.

    1. Make a list of ten compound words that are always written with hyphens.
    2. Write two sentences that include situational compound adjectives that modify nouns.
  2. Write these numbers in words: 42, 89, 265, 1725.
  3. Write these fractions in words: ¾, 7½.
  4. Write three words that each use one of these prefixes and suffixes: all-, ex-, quasi-, -self, -elect, -odd, -something.