Topic Sentences

Learning Objectives

  • Identify topic sentences in paragraphs
Empty flow chart.

Figure 1. A nicely constructed topic sentence will help the reader to better understand the flow of an essay.

Key Sentences

In academic writing, readers expect each paragraph to have a sentence or two that captures its main point. They’re often called “topic sentences,” though many writing instructors prefer to call them “key sentences.” The phrase “topic sentence” could be misleading, because it makes it seem like it always announces the topic of a paragraph or that it is always one sentence, but those things may not always be true. Calling it a “key sentence” reminds us that it expresses the central idea of the paragraph. And sometimes a question or a two-sentence construction functions as the key.

Key sentences in academic writing do two things. First, they establish the main point that the rest of the paragraph supports. Second, they situate each paragraph within the sequence of the argument, a task that requires transitioning from the prior paragraph.

Consider the following examples about epidemiological evidence, meaning evidence related to the study of diseases. Etiological studies refer to the study of the origin of a disease.

Version A:
Now we turn to the epidemiological evidence.

Version B:
If the evidence emerging from etiological studies supports the hypothesis, the epidemiological evidence is also compelling.

Both versions convey a topic; it’s pretty easy to predict that the paragraph will be about epidemiological evidence, but only the second version establishes an argumentative point and puts it in context. The paragraph doesn’t just describe the epidemiological evidence; it shows how epidemiology is telling the same story as etiology. Similarly, while Version A doesn’t relate to anything in particular, Version B immediately suggests that the prior paragraph addresses the biological pathway (i.e., etiology) of a disease and that the new paragraph will bolster the emerging hypothesis with a different kind of evidence (epidemiological). The topic or key sentences make it easy for the reader to keep track of how the essay moves from one paragraph and idea to the next.

Try It: Topic sentences

Read the paragraph about camera flash technology and answer the question below:

When a camera flash is used in a low-light environment, the subject’s eyes may appear red in the finished photograph. What is known as “red-eye” is the result of light from the flash reflecting off the pupils of the eyes. The phenomenon of red-eye can be lessened by using the red-eye reduction feature found on many SLR cameras. This feature activates a lamp that shines a small light directly into the subject’s eyes. When this happens, the diameter of the pupil is reduced, thus tightening the opening in the iris. Since a smaller pupil means a smaller host for the reflection, the chances of red-eye occurring are greatly reduced.

Topic sentences have a topic and an angle, just like thesis sentences. But the angle of topic sentences usually is smaller in range than that of the thesis sentence. Very often the topic remains the same from thesis to topic sentence, while the angle shifts as the writer brings in various types of ideas and research to support the angle in the thesis.

Look at this sample below; these are topic sentences created from the thesis sentence. The topic remains the same in all (regular exercise) and the overall angle remains the same (benefits). But the angle narrows and shifts slightly from topic sentence to topic sentence as the writer brings in different supporting ideas and research.

Thesis Sentence Topic Angle
A regular exercise regime creates multiple benefits, both physical and emotional. Regular exercise Physical and emotional benefits
Topic Sentence Topic Angle
One physical benefit of having a regular exercise regime is longevity. Recent studies have shown that… Regular exercise Physical benefit of longevity
Exercise reduces heart and cholesterol rates when done at least three times per week… Regular exercise Physical benefit of reduced cholesterol
Another physical benefit of regular exercise is that it results in stronger heart and lungs… Regular exercise Physical benefit of stronger heart and lungs
People who exercise regularly have less trouble with sleep disorders… Regular exercise Physical benefit of less trouble sleeping
A benefit that spans the physical and emotional results of regular exercise is the release of endorphins, or substances produced by glands as a byproduct of exercise… Regular exercise Physical and emotional benefits of endorphins
In multiple studies, regular exercise has been shown to reduce stress… Regular exercise Emotional benefit of reduced stress
Because regular exercise often helps to slow the effects of aging and maintain a good body weight, people who exercise regularly experience the emotional benefits of good self-image and self-confidence in their looks… Regular exercise Emotional benefit of better self-image & confidence

Notice that in the examples above, the topic sentence reflects back to the overall thesis (A regular exercise regime creates multiple benefits, both physical and emotional.) and introduces the reader to the specific topic and angle of each paragraph (improving sleep, reducing stress, etc.). An effective topic sentence not only introduces the topic of each paragraph, but it reminds the reader of the overall thesis, driving home the point of the piece of writing.

Realize that all paragraphs do not need topic sentences. Sometimes, you may need multiple paragraphs to help work your way behind the ideas related to one topic sentence because you have a lot of supporting information.

Watch It

You’ll recall that effective annotating includes taking careful notes to extract the main ideas from a reading. In order to find the main ideas, you’ll need to know how to identify the thesis statement and topic sentences in each paragraph. This video will walk you through that process, and also highlight some other key components of essays, like supporting details and transitional words and phrases.

You can view the transcript for “Annotating an Essay or Book” here (opens in new window).

In our next section, we’ll move beyond the topic or key sentence to all the other pieces that make up a strong paragraph, like supporting details and transitional words and phrases.

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