hyphens, compound words, and unit modifiers

Hyphenation rules can be intimidating, but remember that the ultimate goal is to provide as much clarity and consistency as possible. If you can’t find the proper hyphenation in this style guide, consult the AP Stylebook, followed by Merriam-Webster.

1. Verb Phrases: Verb, Noun, and Adjective Forms

Verb phrases that contain an adverb (e.g., build up, set up, start up, and break down) are usually written as two words. The noun and adjective forms of these words are usually one word, although there are exceptions.

We observed the slow buildup of biofouling on the blades.
The algae began to build up.

We helped with the setup.
To set up the experiment, begin with fresh samples.

The startup costs were more than we estimated.
The project is expected to start up next year

I think I'm having another breakdown.
It's time to break down the tent.

2. Compound Words Containing Prefixes and Suffixes

In most cases, you don't need a hyphen between prefixes or suffixes and the root words.


threefold, hundredfold
(also 100-fold)

Sometimes hyphens are needed when the root word begins with the same letter that the prefix ends in.


Use a hyphen between prefixes and proper nouns (but not common nouns) or dates whether they're used as nouns or modifiers.


Use two hyphens when adding a prefix to a word that already contains a prefix, even when there is no hyphen after the prefix in the original word.


Finally, these prefixes usually require a hyphen: "ex," "self," and "quasi."

3. Unit Modifiers With and Without Hyphens

Use a hyphen to indicate that words have been combined into a unit modifier, which is a descriptive expression composed of two or more words that form one new meaning. For example, in the term "flat-plate collector," "flat-plate" is the unit modifier. Here are some examples of unit modifiers that usually include hyphens:

low-level radiation
last-minute addition
high-temperature process
fatigue-induced wear
nine-story building
cost-effective solution

To see how adding the hyphen can prevent confusion, consider these examples:

The scientists tested a new defect causing gas.
The scientists tested a new defect-causing gas.

In the first example, the scientists might seem to have tested a defect; in the second example, it's clear that they tested a gas.

You don't need a hyphen in common unit modifiers that are not ambiguous or confusing.

high school students
solar radiation resource
solar thermal electric systems

Don't use a hyphen when both words of a unit modifier are capitalized.

Biofuels Program objectives
Pacific Ocean exploration
World Cup qualifier

Leave out the hyphens if you rewrite a sentence so the words in the unit modifier come after the noun they describe

We purchased state-of-the-art lab equipment.
We purchased lab equipment that reflects the state of the art.

They made some last-minute adjustments.
They made some adjustments at the last minute.

Don't use a hyphen with a unit modifier containing an adverb ending in "-ly."

heavily skewed results
frequently missed deadlines
commonly seen mistakes

Don’t use a hyphen with two-word Latin phrases when the phrases are used as modifiers.

candidates for in situ testing at wind farms
at the atomistic scale, ab initio models provide detailed descriptions

When you use numbers in unit modifiers, retain all the necessary hyphens. However, do not use a hyphen to join a number and words such as “million” or “billion.”

2-ft-diameter tubes
13-cm-wide substrate
$4 million prize

Or rewrite the sentence to omit the hyphens.

tubes that are 2 ft in diameter
a substrate that is 13 cm wide

Use suspended hyphens when your compound modifier is interrupted by another word, such as part of a list.

light-, medium-, and heavy-duty vehicles
5- and 10-m blades

Suspended hyphens can also be used for different prefixes on the same root word (micro- and mesoscale), but the preferred style is to write out each word fully (microscale and mesoscale). Use your best judgment, considering both the context and audience.

4. Common Hyphenated Terms

Certain words and phrases are always hyphenated, regardless of context. Examples include:

decision-making (but decision maker)