The main idea, or topic, sentence of a paragraph is the sentence that gets to the heart of the subject of that paragraph. The purpose of a main idea sentence is to tie together all the other facts presented in a paragraph to prove the author's point. While elementary students are most often taught that the topic sentence should be the first sentence of the paragraph, it's possible to make the main idea clear to readers in more subtle ways -- without leading with a blanket statement.
Setting the Purpose
The main idea sentence sets the stage for a paragraph. It may help a writer come up with the details of a paragraph first and then decide on a topic sentence that ties everything together. For example, if the essay is about international foods and a specific body paragraph deals with Italian dishes, good writers brainstorm different dishes to write about and then come up with a topic sentence such as "Italian dishes often include various pastas, sauces, and cheeses." By leading with such a sentence, the reader gets a preview of what he will be reading about in the paragraph to follow.
Stated or Implied Topic Sentence
Main idea sentences can be specifically stated or implied by details in the paragraph. In the previous example, the main idea of the paragraph about Italian meals is clearly stated. An implied topic sentence covering the same information can be posed as a question: "Do you enjoy cheese, tomatoes, and noodles? If so, you would love a trip to Italy." Implied main idea sentences may also be conditional sentences: "If you love various cheeses, sauces, and pastas, you would love the meals you eat in Italy." Although it's not explicitly stated, leading with these sentences prepares the audience for the information to come in the paragraph.
Placement of Topic Sentence
Topic sentences fall either at the beginning or end of a paragraph. At times, it's important to lead with a sentence that explains the main idea so the reader knows what details of the paragraph are important. For example, when describing a process to follow, the reader should know that the details will be in sequential order. For example, if a paragraph will describe how to build a model airplane, the reader needs to know what the text to follow will explain.
However, sometimes leaving a topic sentence for the end of a paragraph helps build suspense and ties together seemingly unrelated ideas. An author may choose to barrage the reader with details about a backpacking trip through Europe and wrap the paragraph up by stating, "The opportunities I had over the past summer have helped changed the way I see the world." Again, although not explicitly stated, this paragraph is built to make the reader aware that traveling the world can change her perspective.
Split Topic Sentences
Main idea sentences can be split to convey contrasting viewpoints. Paragraphs using split topic sentences often follow a curve: One viewpoint is stated and supported with details, followed gradually by details that refute the original statement; the paragraph ends with a final refuting statement. For example, an author may state, "Pizza is the greatest food ever created," followed by details of why pizza is so amazing. This would then be followed by details about the consequences of eating only pizza and close with a statement such as "Pizza may be great, but too much of it is certainly not healthy." The two statements "bookend" the details, creating a clear picture of the main idea of the paragraph.
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