You know that supporting details must connect to the main idea. A paragraph has unity when every sentence does. One sentence that drifts can pull the reader off course.
Key idea
In a unified paragraph, every sentence supports the one main idea. A sentence about something else, even an interesting one, belongs in a different paragraph or nowhere.
Why it works
Strunk’s rule 8 (1918) says, “Make the paragraph the unit of composition: one paragraph to each topic.” Readers use paragraphs to sort ideas. When a paragraph mixes two topics, the reader can’t tell which one matters, and the main idea gets weaker.
Worked example: Find the drift
“(1) Penguins are excellent swimmers. (2) Their wings work like flippers. (3) Their bodies are shaped like torpedoes. (4) Some penguins live in warm places, such as the Galápagos. (5) They can stay underwater for several minutes.” Which sentence breaks the unity?
- The main idea: penguins are excellent swimmers.
- Sentences 2, 3 and 5 explain how they swim.
- Sentence 4 is about where penguins live, not how they swim.
Answer: Sentence 4.
Worked example: Cut it or move it?
What should the writer do with the sentence about the Galápagos?
- It is a good fact, but about a different idea: where penguins live.
- If the essay has a paragraph about penguin homes, move it there.
- If not, cut it.
Answer: Move it to a paragraph about where penguins live, or cut it.
Common mistake
Keeping an interesting sentence that drifts from the main idea because it is fun or took effort to find. Check each sentence against the topic sentence, not against the sentence just before it: a paragraph can drift one small step at a time.
In Stay on Topic young writers learn the same idea with pictures. In longer writing, unity applies at two levels: each paragraph sticks to its topic sentence, and each paragraph sticks to the essay’s thesis.