What should be included in a complete thought before a colon?

In written communication, a colon serves to introduce an explanation, a list, a quotation, or further elaboration on the information provided before it. For the usage of a colon to be grammatically correct, what precedes it must be a complete thought—specifically, a grammatically complete sentence that can stand on its own. This means it should contain at least a subject and a verb, and express a full idea.

For example, consider the sentence: "She brought the necessary supplies: pencils, paper, and markers." The portion before the colon, "She brought the necessary supplies," is a complete thought, effectively setting the stage for what follows.

Using an incomplete clause, a single adjective, or a standalone incomplete thought wouldn't provide the clarity and structure necessary when using a colon, which is why those options don't fulfill the requirement needed for proper punctuation.