A compound word is a word that combines two or more distinct words to create a new word with a specific meaning.

Compound Words Definition

1. Closed compounds.

Words like keyboard, football, and notebook are closed compound words. They’re written together as a single word without hyphens or spaces because they’ve been normalized to mean a specific idea.

Other examples include airport, worldwide, birthday, extraordinary, sailboat, and a bunch of other sports terms like baseball, softball, basketball, etc.

2. Open compounds.

Word pairs that form new meanings are open compounds. Words like high school, vice president, middle class, post office, and truck driver are examples of how two words combine to create a new concept.

A few more examples include school bus, decision making, real estate, light year, etc.

3. Hyphenated compounds.

Sometimes when you combine two or more words together, especially when they’re used as an adjective, you should hyphenate them. For example, your sister-in-law or any other in-law is always hyphenated, as is the term on-site.

Here are more examples of hyphenated compounds when used as modifiers to describe the noun that follows them:

• high-speed chase
• part-time employee
• full-time job
• fire-resistant pajamas
• good-looking dog
• well-respected politician
• government-funded program

Compound Words Examples: Exceptions to the Rule

Of course, there are always exceptions to the rules. One thing to keep in mind is that compound words function differently depending on the part of speech they’re used in. Let’s look at an example:

• Carryover can be a noun: "They had extra money thanks to the carryover of last month’s budget." This is a closed compound word used as a noun.
• Carry over can be a verb: "His extra vacation time will carry over to next year’s balance." This is an open compound used as a verb.
• Carryover can be an adjective: "The budget allows carryover funds to be reallocated." This is a closed compound used as an adjective.

Sometimes open compounds should be hyphenated when used as an adjective and not when used as an adverb. For example:

• Adjective: "The full-time teacher works long, hard hours."
• Adverb: "The teacher works the full time permitted to educate his students."

Got that? Here's another example just to make sure:

• Adjective: "The third-grade student reads at a higher level."
• Adverb: "The student is in third grade."

Finally, you never hyphenate compound words with an -ly ending, such as:

• "The heavily decorated general was the parade marshal."
• "The hotly contested political race divided voters."
• "A newly recorded song is climbing the music charts."

Compound Words: Final Advice

Compound words are complex. Your best bet is to look them up in the dictionary where mostly you’ll find straightforward rules.

One of the simplest descriptions is in the Texas Law Review Manual of Style, which reads: "When two or more words are combined to form a modifier immediately preceding a noun, join the words by hyphens if doing so will significantly aid the reader in recognizing the compound adjective." But it’s that last "if" part that leaves a lot up to your judgment. Good luck!