In Tell What It Is About you named the topic of a piece. A paragraph does the same in one sentence. The topic sentence states the paragraph’s main idea, usually right at the start, so the reader knows what every other sentence is there to support.
Key idea
A topic sentence states the paragraph’s main idea: a topic plus what you will say about it. It is broad enough to cover the whole paragraph, and specific enough to need support.
Why it works
Readers understand details faster when they know what the details are for. Strunk’s rule 9 (1918) puts it simply: “As a rule, begin each paragraph with a topic sentence.” The topic sentence is a promise; the rest of the paragraph keeps it.
Worked example: Announcement or topic sentence?
Improve: “This paragraph is about owls.”
- It names the topic (owls) but says nothing about it.
- Ask: what do I want to show about owls?
- Add the point: owls are built to hunt in the dark.
Answer: “Owls are built to hunt in the dark.”
Worked example: Too small to be the main idea
Could “Owls have soft edges on their feathers.” be the topic sentence of the owl paragraph above?
- It is true, but it is one detail.
- It doesn’t cover the eyes or the ears.
- A detail belongs in the body of the paragraph, supporting a bigger point.
Answer: No. It is a supporting detail, not the main idea.
Common mistake
Writing an announcement (“This paragraph is about dogs.” “I am going to tell you about…”) or a small detail as the topic sentence. Check: does it make a point about the topic, and does every other sentence in the paragraph fit under it?
- Too broad: “Animals are interesting.” (Which ones? How?)
- Too narrow: “An owl’s eye is shaped like a tube.” (One detail.)
- Just right: “Owls are built to hunt in the dark.”
In stories and in some essays, a writer may hold the main idea back or leave it implied. For explaining and arguing, a clear topic sentence near the start is the dependable choice.