SAT® · Reading and Writing: Boundaries — Within Sentences
What Must Come Before a Colon
A colon can only follow a group of words that could stand alone as a complete sentence ending in a period. If the words before it can't end a sentence by themselves, the colon is wrong.
A colon announces that something is coming: a list, an explanation, an example. But before it can point forward, what's behind it has to be able to stand on its own. Test it by covering up everything after the colon and asking whether the remaining words could end with a period. 'She packed three things' passes that test; 'She packed' does not, because 'packed' needs an object to feel finished. That's the entire rule, and it never changes no matter what follows the colon.
The mechanism is just that period-swap test, applied mechanically every time. Read only the words to the left. If they form a complete thought, the colon works. If they end on a preposition, a verb that needs more, or a word like 'such as' or 'including', the colon fails, because those words are built to lean on what comes next, not to stand alone. People often think phrases like 'such as' or 'for example' invite a colon, but they do the opposite: they're already doing the pointing-forward job themselves, so a colon right after them is redundant and wrong.
This sits next to the other joint marks in the same family: semicolons, dashes, and commas all have their own left-side and right-side rules, and colons are the strictest about the left side. Once this test is automatic, spotting a bad colon is just as fast as spotting a good one.
Key terms
- Colon
- A punctuation mark that points forward from a complete statement to a list, example, or explanation.
- Independent clause
- A group of words with a subject and verb that can stand alone as a complete sentence.
- Period-swap test
- Checking whether the words before a colon could themselves end with a period.
- Stranded preposition
- A preposition like 'for' or 'to' left without the object it needs, making the clause incomplete.
Watch it explained
- How it worksHow does the colon test reduce to checking whether everything to its left could end with a period?
- How it worksHow does a colon point forward, and why does that direction decide what may sit on each side of it?
- Worked exampleWalk me through why 'The recipe calls for: flour, butter, and salt' misuses its colon.
- Common mistakeWhy isn't a phrase like 'such as' or 'including' a signal that a colon should come next?
Check yourself
End of topic.