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.

2. Compound Words Containing Prefixes and Suffixes

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

Exceptions: 
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:

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

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.

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

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

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

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

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.”

Or rewrite the sentence to omit the hyphens.

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

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: