Is It Babysit, Baby Sit, or Baby-sit? Complete Guide to Compound Words: Open, Hyphenated, and Closed

Babysit or baby-sit? Common sense or common-sense? First aid, first-aid, or firstaid? There are three different kinds of compound words—open, closed, and hyphenated—and it can be tricky to keep them all straight!

Overview: What Is a Compound Word?

A compound word is made up of two or more words working together. They come together to create a single word that has a meaning independent of either of the words that make it up.

For example, firefly is a closed compound word made up of the words fire and fly. Firefly doesn’t mean the same thing as fire or fly (though the firefly insect does have similarities to both of those things) but instead has a separate meaning all its own.

Compound words might be made up of adjectives, nouns, verbs, prepositions, you name it. Sometimes people lump in prefixed words with compound words (e.g., postmodern, antifascist, quasi-official), but they’re technically separate things because prefixes are not complete words on their own.

There are 3 Kinds of Compound Words: Open, Hyphenated, and Closed

If a compound word has a space/spaces between the words that make it up, it’s open (e.g., ice cream). If there’s a hyphen between the words, it’s hyphenated (e.g., deep-fried). If there is no space or hyphen between the words, it’s closed (e.g., buttermilk).

Your best friend in knowing whether a compound word should be open, closed, or hyphenated is the dictionary! I use Merriam-Webster.com, which is free.

Open Compound Words

Open compound words are a little bit tricky, because they look like two different words. However, they’re really acting as a single word.

Here are some examples:

• high school
• sleeping bag
• deviled eggs
• ice cream
• real estate
• living room
• school bus

To pluralize a noun that is an open compound word, you usually only need to pluralize the FINAL word in the compound (not any of the other words). For example, you’d write hot dogs, high schools, and dining rooms, NOT hots dogs, highs schools, and dinings rooms.

A Word of WARNING: Be careful with open compound words because sometimes they take a hyphen if they are functioning as an adjective before another noun. Here are a couple of examples:

The medical student administered first aid.

BUT

The medical student asked a bystander to bring the first-aid kit. (Here, “first-aid” is functioning as an adjective for “kit.”)

NOT

The medical student asked a bystander to bring the first aid kit.

My favorite genre is science fiction.

BUT

I’m going to the bookstore to get a new science-fiction book. (Here, “science-fiction” is functioning as an adjective for “book.”)

NOT

I’m going to the bookstore to get a new science fiction book.

Adding a hyphen (or multiple hyphens) to the open compound word signals to the reader that the compound word is functioning as an adjective and not a noun. Otherwise, it could get confusing to have a bunch of nouns strung together without an indication of how they’re functioning in the sentence.

Hyphenated Compound Words

Although open compound words SOMETIMES have hyphens (when they’re functioning as adjectives before a noun), hyphenated compound words ALWAYS have hyphens.

Here are some examples of hyphenated compound words:

• mass-produced
• mother-in-law
• to roller-skate (BUT note that the noun roller skate is an OPEN compound word—only the verb to roller-skate is always hyphenated)
• editor-in-chief
• clean-cut
• know-it-all
• over-the-counter

Some numbers, when spelled out, are also hyphenated compound words. Most sources say you should hyphenate numbers 21 through 99:

• forty-nine
• thirty-two
• seventy-eight

For numbers higher than 99, you don’t need to hyphenate:

• one hundred and three
• eight million

However, if a number between 21 and 99 is part of a bigger number, you should still hyphenate that part of the word:

• one hundred and twenty-two
• seven million and forty-eight
• eighty-four billion

Pluralization with hyphenated compound words can be tricky. You don’t always pluralize the last word—for example, it’s mothers-in-law, NOT mother-in-laws. Use the dictionary to find the correct pluralization for each word.

Closed Compound Words

A closed compound word is still made up of two or more words, but there are no spaces between those words.

Here are some examples:

• smartphone
• birthrate
• cheeeseburger
• milkshake
• weekend
• babysitter
• snowball
• without

So there’s your answer to the question posed in this post’s title: it’s babysit, NOT baby-sit or baby sit!

Closed compound words are much easier to use compared to open and hyphenated ones. Without spaces, these words both look and function as individual words!

Permanent vs. Temporary Compound Words

Not all compound words are in the dictionary because sometimes we make them up for a specific situation.

Take this sentence for example:

The textbook-wielding professor marched to the front of the classroom.

If you look up textbook-wielding in the dictionary, you will find absolutely nothing (trust me—I checked). That’s because it’s a temporary compound word that I created just for this sentence.

Just because it’s not in the dictionary doesn’t mean it’s not a legitimate word. Making up compound words is totally OK! Usually, temporary compound words are hyphenated (rather than open or closed).

Permanent compound words have been accepted into the English vocabulary and are found in the dictionary. Sometimes, temporary compound words that are used frequently enough eventually become permanent and are added to the dictionary!

The Eventual Fate of Open and Hyphenated Compound Words

There’s a trend in the English language for all open or hyphenated compound words to eventually become closed. In fact, the life of a compound word usually starts out as open, then becomes hyphenated with frequent enough use, then finally becomes closed when we get comfortable enough with the combination of words.

Take online, for example. At first, this word started as on line. Then, it turned into on-line. Now, we’re all using online. If I try to type “on-line” into Merriam-Webster.com, it automatically corrects it to “online.”

So as a compound word is used more and more often, it starts to change forms. The end form is always a closed compound word.

This doesn’t always happen at the same time for closely related compound words. For example, birthrate has become a closed compound word, but death rate is still an open compound word.

If you have any questions about compound words, drop them in a comment and I’ll find an answer for you!

Sources:

• The Chicago Manual of Style. 17th ed. Chicago, IL: The University of Chicago Press, 2017.
• Ebbitt, D. R., and W. R. Ebbitt. Index to English. 8th ed. New York: Oxford University Press, 1990.
• Ellis, Matt. “Compound Words: Open, Closed, or Hyphenated?” Grammarly, November 11, 2022. https://www.grammarly.com/blog/grammar/open-and-closed-compound-words/.