Hyphenated Compounds: Part I

by Tina Blue
January 15, 2001

I have been asked to address the problem of hyphenating compound words. This article is the first in a series that will take up the principles of hyphenation. By the way, if you request an article or series of articles from me, please indicate whether you would like me to mention your name and add a link to your website. If you would, I will.

A compound word is a combination of two or more words that serves the purpose of a single part of speech.

There are three possible ways of rendering compound words: they can be written separately (prime minister, high school, vacation home); they can be hyphenated (all-day event, used-car dealer, three-week vacation, tongue-lashing, know-it-all attitude); or they can be written as one word (highway, spaceship, boyfriend, racehorse, onsite, offsite, the Australian outback).

There is a wide range of variation in the use of hyphens to join compound words. No rules govern all combinations, and the possible combinations are virtually limitless, so many of them will not be found in the dictionary. Furthermore, even dictionaries vary in their treatment of some compound words.


Some compounds are so common and so frequently used that they have become "permanent compounds." Combinations like racehorse, boy scout basketball, railroad, prime minister, vacation home, know-it-all, sit-in, city-state, high school, break-in, and breakout can be looked up in a current dictionary to determine whether to separate them, hyphenate them, or write them as one word.

And even though some dictionaries may vary in their treatment of certain permanent compounds, at least the presence of one form or another in an authoritative dictionary provides justification for the style the writer finally settles on.

Once you have selected a style for a given compound word, however, it is essential that you use the form consistently within the body of that particular piece of writing. It will not do, for example, to write "science-fiction writers" in one paragraph, and then write "science fiction writers" two or three paragraphs later.

Of course, it would be perfectly acceptable to write "science-fiction writers," and then write "science fiction" later in the same piece, because in the first instance "science-fiction" is a compound adjective modifying the noun "writers," whereas in the second, the compound consists of the adjective "science" modifying the noun "fiction." Because the grammatical structures are different, they are not governed by the same rule of usage.

EXAMPLE:  Whatever science-fiction writers write tends to be labeled science fiction, even if it would fit better into the category of fantasy.


Some compounds are created on the spot to fulfill a specific need (which is to say, they are on-the-spot creations). Permanent compounds often start out as improvised compounds, but then become so widely accepted that they are included in the dictionary as permanent compounds.

Examples of temporary compounds that have made the transition to permanent compounds are know-it-all, heart-stopping, sit-in, and down-in-the-heels. But even these compounds are hyphenated or not depending on the context of the sentences they occur in.


~That is a really off-the-wall suggestion.

~That suggestion is really off the wall.

~We should get rid of these out-of-date textbooks and replace them with more                up-to-date materials.

~Those textbooks are really out of date. The last time they were up-to-date was in          1949.

Notice that "out of date" can be left unhyphenated following the linking verb "are," but "up-to-date" still seems to require the hyphens. On the other hand, it would also be acceptable to write "These textbooks are really out-of-date," simply because that usage is so common, though usually a modifying compound is not hyphenated following the noun it modifies or following a linking verb ("to be" or "state of being" verb).

Besides, it would seem inconsistent to write "out of date" without hyphens following the linking verb when "up-to-date" is hyphenated in a similar context. Therefore, in the preceding example, it would be better (more consistent) to write "These textbooks are really out-of-date. The last time they were up-to-date was in 1948."

Most on-the-spot, improvised combinations, even some that are widely used (some widely-used ones), are not to be found in the dictionary, so writers must struggle to apply sometimes variable rules in order to achieve a reasonable level of consistency in their treatment of compound words.

Although there are exceptions to most of the rules governing compound words, there are some generally applicable principles, and some rules that are more reliably applicable than most.

For the remainder of this article I will present those rules that you can most comfortably rely on, without worrying about too many lurking exceptions.  In "Hyphenation: Part II (Prefixes)," I deal  with rules governing hyphenation with common prefixes, and in a third article, "Hyphenation: Part III (Suspensive Hyphens),"  I deal with a fairly rare use of hyphens in paired or sequenced compounds.


1. The parts of written-out compound numerals from twenty-one to ninety-nine are joined by a hyphen.


2. A hyphen joins the numerator and denominator of a written-out fraction.


BUT when a hyphen already appears in either the numerator or the denominator, the hyphen between the numerator and denominator is omitted.

twenty-four thirty-fifths
three ten-thousandths
two twenty-fifths

3. When a written-out number or a numeral is joined to a unit of measurement and the resulting compound is used as an adjective, use a hyphen to join the number and its unit of measurement.

the 100-yard dash
a ten-day tour
a two-minute speech
a 40-hour work week

4. With other types of phrases with numbers, use hyphens to join the parts of the numerical modifier.

a nine-year-old girl
a six-year-old
a five-dollar bill
an eighteenth-century philosopher
a 20th-century novelist

5. A hyphen can be used to indicate a range of numbers, if an n-dash is not available.

~The tour was 2-3 hours long.
~The U.S. was part of the Allied war effort in the years 1941-1945.

BUT if the phrase is written as "from . . . to" or "between . . . and,"  then the hyphen is not used to replace the second word.

WRONG: --The tour was between 2-3 hours long.
RIGHT: --The tour was between two and three hours long.

WRONG: --The U.S. was part of the Allied war effort in the years from 1941- 1945.
RIGHT: --The U.S. was part of the Allied war effort from 1941 to 1945.

WRONG: --The lead actors were from 12-14 years old.
RIGHT: --The lead actors were from 12 to 14 years old..
RIGHT: --The lead actors were between 12 and 14 years old..
RIGHT: --The lead actors were 12-14 years old.


NOTE: The second article in this series on hyphenation is "Hyphenation: Part II (Prefixes)" and the third is "Hyphenation: Part III (Suspensive Hyphens)."
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