Compound Words: Definition, Types, Hyphenation Rules, and Common Errors

What Is a Compound Word?

A compound word is two or more words combined to express a single concept. The combined word often has a meaning that goes beyond the sum of its parts. A blackbird is a specific type of bird, not just any bird that's black. A greenhouse isn't simply a house that's green. It's a structure with a specific function.

Compound words make language more efficient. They let writers combine ideas into a single term instead of using descriptive phrases. A "raincoat" is faster and clearer than "a coat that protects against rain." A "bookshelf" is more direct than "a shelf for books."

English is unusually productive in forming compound words. New compounds enter the language regularly as new concepts emerge. "Smartphone," "podcast," and "selfie" are all compounds that didn't exist a generation ago. Writers who understand how compound words work can use them confidently in their own writing. They can also recognize when an unfamiliar combination is a legitimate compound or simply two adjacent words.

The Three Types of Compound Words

English compound words fall into three categories based on how they're written. Each type follows different conventions, and the same word can sometimes appear in different forms across different style guides.

1. Closed compounds (written as one word)

Closed compounds combine two words into a single unit with no space or hyphen. These are the most common type in everyday English. Examples include:

• notebook
• sunflower
• toothbrush
• bedroom
• airport
• keyboard
• firefighter
• newspaper
• website
• backpack

Many closed compounds started as open or hyphenated compounds and merged over time. "Online" was once "on-line." "Email" was once "e-mail" and is still hyphenated in some style guides. This drift toward closed forms is a long-running pattern in English.

2. Hyphenated compounds (joined by a hyphen)

Hyphenated compounds join two or more words with a hyphen. The hyphen signals that the words function as a single unit. Examples include:

• mother-in-law
• editor-in-chief
• well-known (when used as a modifier before a noun)
• off-the-cuff
• state-of-the-art (when used as a modifier)
• twenty-one (and other compound numbers from twenty-one to ninety-nine)
• self-aware
• ex-husband

3. Open compounds (written as separate words)

Open compounds are two words that function as a single concept but are written with a space between them. Examples include:

• ice cream
• post office
• high school
• real estate
• full moon
• peanut butter
• living room
• swimming pool

Open compounds can be tricky because they look like two separate words but function as one concept. "High school" refers to a specific kind of institution, not just any school that happens to be tall. The two words act as a unit even though they're written separately.

When to Hyphenate Compound Modifiers

The hardest compound word question for writers isn't whether sunflower is one word or two. It's whether to hyphenate phrases like "well known," "high quality," "long term," and "state of the art." These are compound modifiers, and they follow a specific rule.

The basic rule

Hyphenate a compound modifier when it appears before the noun it modifies. Don't hyphenate it when it appears after a linking verb. Examples:

• She is a well-known author. (Before the noun "author," so hyphenated.)
• The author is well known. (After the linking verb "is," so no hyphen.)
• This is a high-quality document. (Before "document," so hyphenated.)
• The document is high quality. (After "is," so no hyphen.)
• She made a long-term commitment. (Before "commitment.")
• The commitment is long term. (After "is.")

This rule applies to most two-word compound modifiers in English. It's one of the most consistently violated punctuation rules in business and academic writing, which is why editors flag it routinely.

Exceptions to the rule

A few categories of compound modifiers don't take a hyphen even when they appear before the noun.

Adverbs ending in -ly. When the first word is an -ly adverb, no hyphen is needed.

• A highly trained editor. (Not "highly-trained.")
• A carefully written report. (Not "carefully-written.")
• A poorly worded sentence. (Not "poorly-worded.")

Proper nouns. Compound modifiers built on proper nouns generally don't take a hyphen.

• A New York apartment. (Not "New-York apartment.")
• A Wall Street investor. (Not "Wall-Street investor.")

Foreign phrases. Compound modifiers in foreign phrases that English has adopted whole usually aren't hyphenated.

• An ad hoc committee. (Not "ad-hoc committee.")
• A bona fide offer. (Not "bona-fide offer.")

How Style Guides Differ on Compound Words

Different style guides give different guidance on compound word treatment. Writers should know which guide their work follows.

Chicago Manual of Style

Chicago is the standard guide for book publishing, academic writing in the humanities, and many magazines. Chicago provides a detailed "hyphenation guide" that lists hundreds of compound forms and tells writers when each one should be open, closed, or hyphenated. Chicago tends to be conservative, retaining hyphens longer than other guides.

AP Stylebook

AP is the standard for journalism and most consumer publications. AP tends to use fewer hyphens than Chicago, favoring closed or open forms when the meaning is clear. AP also has specific rules for compound words involving prefixes and suffixes that differ from Chicago.

APA Publication Manual

APA is the standard for psychology, education, and many social sciences. APA's compound word guidance generally aligns with Chicago, but APA emphasizes consistency within a manuscript. If you use a closed form in one section, use the same form throughout.

When guides disagree

For words that two guides treat differently, the rule is simple: follow the guide your publication or institution requires. A journalist writing for AP-styled outlets uses "email" as one word with no hyphen. A scholar writing for a Chicago-styled academic press might use "e-mail" or "email" depending on the specific guidance from the press. The most important consideration is consistency within a single document.

Common Compound Word Mistakes Writers Make

• Forgetting to hyphenate compound modifiers before a noun. The most common error is writing "a long term goal" instead of "a long-term goal." Reviewers catch this immediately.
• Hyphenating after a linking verb. The mirror error is "the goal is long-term." This should be "the goal is long term" with no hyphen.
• Hyphenating after an -ly adverb. "A highly-trained editor" is wrong. The correct form is "a highly trained editor" with no hyphen.
• Treating an open compound as one word. Writing "icecream" or "highschool" is a common error in casual writing. These are open compounds.
• Inconsistent treatment within a document. Writing "decision-making" in one paragraph and "decision making" in another is the kind of inconsistency that editors flag. Pick one form and use it throughout.
• Hyphenating proper nouns in compound modifiers. "A New-York apartment" looks wrong because proper nouns generally don't take hyphens in compound modifiers.
• Missing hyphens in compound numbers. Compound numbers from twenty-one through ninety-nine are hyphenated when spelled out: "twenty-one" not "twenty one."
• Hyphenating prefixes that don't need one. Most prefixes attach directly to the base word with no hyphen: "preexisting," "nonprofit," "antiwar." Hyphens after prefixes are used in specific cases. The base word might be a proper noun (anti-American). The prefix might create an awkward double letter (re-elect).

A Sample of Closed Compound Words

These compounds are written as a single word in standard English. The list illustrates how productive English is in forming closed compounds:

• airport, backpack, bedroom, bookshelf, breakfast
• cupcake, daydream, earthquake, firefighter, fireworks
• football, grasshopper, haircut, handwriting, headphones
• homework, jellyfish, keyboard, keychain, lifeguard
• lighthouse, mailbox, moonlight, newspaper, notebook
• playground, postcard, railroad, raincoat, rattlesnake
• seashell, seashore, snowboard, snowman, starfish
• sunflower, sunglasses, sunrise, sunset, tablecloth
• teacup, toothbrush, underwater, waterfall, website
• windmill, woodwork

A Sample of Hyphenated Compound Words

• brother-in-law, sister-in-law, mother-in-law, father-in-law
• editor-in-chief, commander-in-chief, attorney-at-law
• well-known, well-being, ill-advised, ill-equipped
• off-the-cuff, state-of-the-art, run-of-the-mill (when used as modifiers)
• self-aware, self-conscious, self-employed
• ex-husband, ex-president, ex-employee
• twenty-one, thirty-five, sixty-seven (and other compound numbers)
• long-term, short-term, full-time, part-time (when used as modifiers)

A Sample of Open Compound Words

• ice cream, peanut butter, real estate
• high school, middle school, post office
• living room, dining room, swimming pool
• full moon, half hour, coffee table
• cell phone, web page, search engine
• credit card, bus stop, train station
• fire engine, police officer, public school

Why Compound Words Matter for Your Writing

Compound word errors are some of the most visible mistakes in professional writing. They appear in business proposals, academic papers, marketing copy, and journalism. Reviewers and copy editors catch them quickly. The errors signal to readers that the writer may not have engaged carefully with the conventions of the language.

The hyphenation rule for compound modifiers is the single highest-impact compound word rule to master. Writers who get it right look polished and careful. Writers who get it wrong look careless, even when the rest of their writing is strong. For style and grammar questions that come up alongside compound word usage, see our article on comma rules for writers. We also have articles on commonly confused word pairs: its versus it's, further versus farther, and affect versus effect.

Get Your Writing Reviewed by a Professional Editor

Compound word errors are exactly the kind of small detail that benefits from a second pair of trained eyes. Writers often miss their own hyphenation mistakes because the errors look correct individually. The problem only becomes obvious when a reader applies the grammar rule to the sentence.

Editor World's professional proofreading services and academic editing services connect writers with verified native English editors from the US, UK, and Canada. Every editor has been screened for grammar, style guide knowledge, and discipline-specific expertise. Editors flag compound modifier hyphenation issues, style guide inconsistencies, and other compound word errors as part of standard proofreading.

A free sample edit is available from any editor before you commit. Browse editor profiles by subject expertise and verified client ratings to find someone whose background matches your work.

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Page last reviewed: May 2026. Content reviewed by Editor World editorial staff. Editor World, founded in 2010 by Patti Fisher, PhD, is a professional human-only writing, editing, and proofreading marketplace serving writers, researchers, and professionals worldwide. BBB A+ accredited since 2010 with 5.0/5 Google Reviews and 5.0/5 Facebook Reviews.