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.

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:

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.

A last thing to note about key sentences is that academic readers expect them to be at the beginning of the paragraph. (The first sentence in this paragraph is a good example of this in action!) This placement 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. There’s a good chance that many of those first sentences will be key or topic sentences.

However, keep in mind that this isn’t a rule. Sometimes the paragraph’s purpose in a larger piece of writing necessitates that its topic sentence occur elsewhere.