A second function is to give the reader an indication of how the writer will deal with the topic, whether it be by definition, comparison, examplification, analysis, argument, or in some other way.
A topic sentence will frequently pose an assertion which prompts the reader to ask, "Why do you say that? What are your reasons? What is your proof? Explain what you mean. Give me an example." This is the case not just in argument where the writer is trying to convince the reader to accept a point of view, or in persuasion where he is trying to get the reader to pursue a course of action; it is also common in exposition where the writer presents ideas that are novel or difficult, and the reader expects him to give facts and reasons to explain what he means.
At times a topic sentence may also make a transition from the preceding paragraph to the new one. When the topic sentence performs this function, it uses a transitional word or phrase, points back to the preceding paragraph by the repetition of a key term, and then points forward to the new paragraph by introducing a word or phrase that identifies the new subject.
Like the main idea sentence of an essay which may give a preview of the content by listing the subjects to be covered, a topic sentence may list the subtopics of the paragraph, or at least alert the reader as to how many subtopics there are. And a topic sentence that introduces a new term may also define that term.
A topic sentence most often appears at or near the beginning of a paragraph. This is the general practice in exposition where the primary concern is to convey information in a clear and efficient manner. Here, the billboard function of the topic sentence is paramount. Placing the topic sentence near the beginning gives the paragraph a deductive organization that aids comprehension by alerting the reader as to what he can expect to learn.
However, topic sentences can sometimes be placed at the end of paragraphs to good effect. When the topic sentence comes near the end, the paragraph has inductive organization. This is a common when the paragraph provides examples of a phenomenon or involves an analysis. The topic sentence at the end may state a conclusion drawn from the examples presented or from the line of reasoning laid out. Inductive organization is also found in some types of narrative, as for example in fables, where the main idea at the end gives explicit expression to the point that the writer wants the reader to remember.
Sometimes, especially in academic writing, a topic sentence becomes a "topic paragraph." This is often the case when a text book heading is followed by a short paragraph that names the subtopics that will be introduced in that section. Thus, the paragraph previews the information by listing it in the order in which it will be covered. Examples of various types of topic sentences can be seen here.
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