6.2: Key Sentences (a.k.a. Topic 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.” There are at least two downsides of the phrase “topic sentence.” First, it makes it seem like the paramount job of that sentence is simply to announce the topic of the paragraph. Second, it makes it seem like the topic sentence must always be a single grammatical sentence. 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.

The key to staying on topic within a paragraph is starting with a topic sentence. It doesn’t even have to be perfect to work from it! Just figure out what you really want to say in that one specific paragraph and go. Then, you … EDIT (haha, made you flinch!) All joking aside, editing really is a very important step in this process. By going back over what you wrote, you can check to see if what you wrote in that paragraph fits with what you actually intend to say as well as to make sure everything is cohesive and coherent!

Kaethe Leonard

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 these two examples: ²

Version A:

Now we turn to the epidemiological evidence.

Version B:

The epidemiological evidence provides compelling support for the hypothesis emerging from etiological studies.

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. As a reader, it’s easy to keep track of how the paragraph about cells and chemicals and such relates to the paragraph about populations in different places.

By clearly establishing an essential point within its analytic context, a well written key sentence gives both you and your reader a firm grasp of how each point relates. For example, compare these two sets of key sentences, each introducing a sequential paragraph³:

Version A:

At the beginning of the AIDS epidemic, the cause of the disease was unclear. …

The cause of AIDS is HIV. …

There are skeptics who question whether HIV is the cause. …

Version B:

At the beginning of the AIDS epidemic, the cause of the disease was unclear, leading to a broad range of scientific speculation. …

By 1986 HIV had been isolated and found to correlate almost exactly with the incidence of AIDS. …

HIV skeptics, on the other hand, sought to discredit claims based on epidemiology by emphasizing that the pathogenesis of HIV was still unknown. …

Version A isn’t wrong per se; it just illustrates a lost opportunity to show the important connections among points. Both versions portray a process unfolding over time: initial uncertainty followed by a breakthrough discovery and then controversy. Even with the same substantive points, a person reading Version A would have to work harder to see how the material in the paragraphs connects. Readers experience Version B as clearer and more engaging.

Thinking of key sentences as sequential points in an argument reminds one that a key sentence doesn’t have to always be a single declarative one. Sometimes you need two sentences together to achieve the work of a key sentence, and sometimes a question or quotation does a better job than a declarative sentence in clarifying a logical sequence:

Version C:

At the beginning of the AIDS epidemic the cause was unclear. Virologists, bacteriologists, immunologists, and epidemiologists all pursued different leads, reflecting their particular areas of expertise…

If drug use, lifestyle, and “immune overload” didn’t cause AIDS, what did?…

“I’ve asked questions they apparently can’t answer,” claimed retrovirologist Peter Duesberg⁴ who became an oft-quoted skeptical voice in media accounts of AIDS research in the mid-1980s. …

Version C is based on the same three sequential points as Versions A and B: (1) the cause of AIDS was initially unclear (2) HIV was accepted as the cause (3) lone dissenters questioned the claims. However, versions B and C have much more meaning and momentum, and version C, depending on the nature of the argument, features more precise and lively stylistic choices. Opening the second paragraph with a question (that then gets answered) carries forth the sense of befuddlement that researchers initially experienced and helps to convey why the discovery of HIV was a hugely important turning point. Using the self-glorifying Duesberg quote to launch the third paragraph makes the point about lingering skepticism while also introducing a portrait of a leading figure among the skeptics. While Version B is effective as well, Version C illustrates some of the more lively choices available to academic writers.

A last thing to note about key sentences is that academic readers expect them to be at the beginning of the paragraph.⁵ That helps readers comprehend your argument. To see how, try this: find an academic piece (such as a textbook or scholarly article) that strikes you as well written and go through part of it reading just the first sentence of each paragraph. You should be able to easily follow the sequence of logic. When you’re writing for professors, it is especially effective to put your key sentences first because they usually convey your own original thinking, which, as you’ve read here, is exactly what your instructors are looking for in your work. It’s a very good sign when your paragraphs are typically composed of a telling key sentence followed by evidence and explanation.

Knowing this convention of academic writing can help you both read and write more effectively. When you’re reading a complicated academic piece for the first time, you might want to go through reading only the first sentence or two of each paragraph to get the overall outline of the argument. Then you can go back and read all of it with a clearer picture of how each of the details fit in.⁶ And when you’re writing, you may also find it useful to write the first sentence of each paragraph (instead of a topic-based outline) to map out a thorough argument before getting immersed in sentence-level wordsmithing. For example, compare these two scaffolds. Which one would launch you into a smoother drafting process?:⁷

Version A (Outline Of Topics):

1. Granovetter’s “Strength of weak ties”
   1. Definition
   2. Example—getting jobs
2. Creativity in social networks
   1. Explanation
   2. Richard Florida’s argument
3. Implications
   1. For urban planners
   2. For institutions of higher education

Version B (Key-Sentence Sketch):

The importance of networking for both career development and social change is well known. Granovetter (1973) explains that weak ties—that is, ties among acquaintances—are often more useful in job hunting because they connect job-seekers to a broader range of people and workplaces. …

Subsequent research in network analysis has shown that weak ties can promote creativity by bringing ideas together from different social realms. …

Richard Florida (2002) argues that cities would do well to facilitate weak ties in order to recruit members of the “creative class” and spur economic development. …

Florida’s argument can inspire a powerful new approach to strategic planning within colleges and universities as well. …

As you can see, emphasizing key sentences in both the process and product of academic writing is one way to ensure that your efforts stay focused on developing your argument and communicating your own original thinking in a clear, logical way.

A good paper has cohesion. I love outlines, so I really like the idea of writing my first sentence of each paragraph as my plan. This way, you know what to write about and you know that your paper will flow easily. As a reader, this is an important characteristic to me. If the paragraphs are just jumping around in all different directions, I quickly lose interest in trying to follow along. The reader should not have to struggle to follow your paper. Flow can make the difference between an okay paper and a scholarly product.

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