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Copyright 2016 © Laraine Flemming.
An instructor recently pointed out to me that her students were having a difficult time understanding the difference between the terms topic sentence and main idea. Because this confusion comes up a lot in reading courses, I thought it was time to spell out the difference in some detail, first from the writer's perspective and then from the reader's.
While it's true that some textbook authors use topic sentences a lot—authors of business texts use them repeatedly and tend to make them the first sentence of the paragraph—other authors use them more sparingly. Particularly in fields like history, writers frequently structure their paragraphs to suggest or imply a main idea without stating it.
Viewed from the writer's perspective, then, the topic sentence is a verbal device: A writer may (or may not) use it to guide the reader's understanding of the thought communicated by the paragraph. Its status as a writer's tool—to be used heavily or sparingly, depending on the writer's style and purpose—may be one reason why the term made its first appearance in mid-nineteenth century in a composition text, rather than in a reading text. As early as 1862, topic sentences were recommended as a way of guiding the reader through the writer's explanation or argument.
Well-constructed topic sentences are the writer's way of announcing to readers, "This is a general, summary version of what I am trying to get at here. Use it as a stepping stone to guide your understanding of all the other details provided, so that your idea about the paragraph's main idea matches up with mine when you finish reading." No wonder, then, that many textbooks and teachers of writing still tell student writers to use topic sentences whenever possible. It's simple: Clear, well-written topic sentences make the reader's job easier.
But the fact remains that the topic sentence a writer supplies in order to help the reader follow a train of thought is not the same thing as the main idea the reader re-constructs while reading a paragraph. As soon as readers look at a text, they are working to re-create the author's main idea, or intended message. It doesn't matter if the paragraph contains a topic sentence or not, re-creating the author's intended main idea in their own mind is always a central goal for readers.
Readers therefore should not be holding off thinking about the author's point until they locate the topic sentence. Rather they should, from the beginning, be looking at the verbal clues left by the writer and trying to figure out how these clues all add up. If they find a topic sentence that fits the main idea they are mentally building, they know they are on the right track. They also know they can focus more on figuring out how the rest of the paragraph fits in with or develops that point.
But even if the topic sentence never turns up, readers' efforts aren't wasted. They have still completed the major task of comprehension: coming up with a personal, yet still similar, version of the author's main idea.
Useful as the topic sentence might be for communication, readers don't store the language of the topic sentence verbatim, or word for word, in long-term memory. To put it another way, readers don't memorize topic sentences.
Instead, readers store their own mental version of the main ideas called up by the author's words. On some level, readers know what those who study communication via language (called discourse analysts) point out: Words are containers for our thoughts. And those thoughts can almost always be "contained" in various ways.
It's no surprise, then, that the language of the main idea the reader creates from the text might be quite different—and probably a lot more fragmented—than the writer's more formal topic sentence. What's important, though, is that the reader's version of the main idea matches up with the author's main idea, which—as the example that follows illustrates—gets developed via the entire paragraph, not just through an isolated topic sentence.
Because the difference between a topic sentence and the main idea is best illustrated with a real piece of text, let's look closely at the following paragraph (the topic sentence has been underlined):
As the old saying goes, "war is hell," but that expression is particularly appropriate to World War I, when the French, the Germans, and then the British first introduced poison gas into the arsenal of war. Although many people think it was the Germans who first used gas as a weapon of warfare, it was the French who first deployed tear gas in the first month of the war, August, 1914. It was, however, the Germans who upped the ante using poison gas on a large scale at the beginning of the second battle of Ypres.* On the morning of April 22nd, 1915, French and Algerians troops were puzzled by a strange yellow-green cloud moving slowly toward them. Their initial thought was that the cloud was a smoke screen, hiding the advance of German soldiers. But as soon as the cloud enveloped them, the soldiers knew they were wrong about what it hid. Within seconds of inhaling the chlorine gas the cloud contained, the men were choking and rolling on the ground in a desperate attempt to breathe. Many died and even those who escaped suffered lung damage. Once the Allies realized what had happened, they too began using poison gas, with the British being the first to launch some 400 canisters of chlorine gas on September 25, 1915, and the use of poison gas, with all its horrific and deadly effects, continued to escalate throughout the war. (Source of information)
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*Ypres: a small town in Belgium
Anyone hoping to remember the point of this paragraph will not try to mentally record the exact wording of the opening topic sentence. In fact, research shows that the words of a topic sentence can be changed, but the reader's mental version of it will remain the same, as long as the idea stated in the revised topic sentence is similar to the idea expressed in the first. It's as if the reader were saying to the writer, "I'm not going to fool around with an idea I already understand just because you can't make up your mind about what words you should use to express it."
What that means is this: Experienced readers don't memorize topic sentences. They remember the main ideas the topic sentences express. And they remember as well the way in which those ideas get developed within paragraphs.
While the topic sentence in the above paragraph helps focus the reader's attention, both by its content and its location, it would not be recorded in long-term memory. What would end up in memory is a main idea expressed something like this: "World War I was the first war to use poison gas, starting with the French using tear gas and then the Germans and British using deadly chlorine gas" or like this: "World War I saw the first use of poison gas, starting with the French using tear gas and the Germans and British using deadly chlorine gas that left soldiers unable to breathe." And those are only two of several possibilities.
Different readers are bound to formulate differently-worded versions of the main idea, although the meanings would and should be similar.
Note, too, how the sample main ideas likely to be stored in memory don't even mention the opening clause in the topic sentence—about war being hell and World War I being particularly hellish. That's because the paragraph focuses on the introduction of poison gas into warfare. The paragraph is not about the writer's attempt to prove that the expression "war is hell" fits World War I better than World War II or any other war that came after it. The opening of the topic sentence is more about the writer's wish to stir up the reader's interest than an attempt to convey crucial information. In other words, like the topic sentence itself, the opening is a verbal device employed by the writer in order to engage the reader.
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Last update of this page: April 11, 2016