The colon is like a sign on the highway, announcing that something important is coming. It acts as an arrow pointing forward, telling you to read on for important information. A common analogy used to explain the colon is that it acts as a flare in the road, signaling that something meaningful lies ahead.

The colon is not just used to introduce a list; it is far more flexible. The colon can appear after the first word(s) of a sentence, just before the final word(s) of a sentence, or even between two independent clauses. It can be used to provide emphasis, to explain, or to summarize. Thus, it is one of the most powerful punctuation marks.

Use the colon when you wish to provide emphasis.

• To address this problem, we must turn to one of the biologist’s most basic tools: the petri dish.
• My grandfather, a research scientist, gave me some critical advice about petri dishes: don’t drop them.

Use the colon to introduce material that explains or summarizes what has preceded it.

• The petri dish: one of the biologist’s most basic tools.
• In low-carbon steels, banding tends to affect two properties in particular: tensile ductility and yield strength.
• The research is conclusive: global warming is really happening.

Use the colon to present a list or series, particularly when there is a lot of similar material to join or when the items in the list include commas. When you are using a colon in this way, the colon usually gives the idea of “as follows” or “which is/are.” That means you should not include a colon after words like “for example,” or when you may be separating a verb from its object.

• A compost facility may not be located in the following: within 300 feet of an exceptional-value wetland, within 100 feet of a perennial stream, or within 50 feet of a property line.
• A backyard compost pile can process many items: fruit and vegetable scraps; crushed egg shells; spoiled soy, rice, almond, or coconut milk; loose leaf tea.
• Incorrect sentence: The three primary colors are: red, blue, and yellow. (The colon is not necessary to introduce this list because the first part of the sentence is grammatically incomplete without including the colors.)