What is a topic sentence?
A topic sentence states the main point of a paragraph: it serves as a mini-thesis for the paragraph. You might think of it as a signpost for your readers—or a headline—something that alerts them to the most important, interpretive points in your essay. When read in sequence, your essay's topic sentences will provide a sketch of the essay's argument. Thus topics sentences help protect your readers from confusion by guiding them through the argument. But topic sentences can also help you to improve your essay by making it easier for you to recognize gaps or weaknesses in your argument.
Where do topic sentences go?
Topic sentences usually appear at the very beginning of paragraphs. In the following example from Anatomy of Criticism, Northrop Frye establishes the figure of the tragic hero as someone more than human, but less than divine. He backs up his claim with examples of characters from literature, religion and mythology whose tragic stature is a function of their ability to mediate between their fellow human beings and a power that transcends the merely human:
The tragic hero is typically on top of the wheel of fortune, half-way between human society on the ground and the something greater in the sky. Prometheus, Adam, and Christ hang between heaven and earth, between a world of paradisal freedom and a world of bondage. Tragic heroes are so much the highest points in their human landscape that they seem the inevitable conductors of the power about them, great trees more likely to be struck by lightning than a clump of grass. Conductors may of course be instruments as well as victims of the divine lightning: Milton's Samson destroys the Philistine temple with himself, and Hamlet nearly exterminates the Danish court in his own fall.
The structure of Frye's paragraph is simple yet powerful: the topic sentence makes an abstract point, and the rest of the paragraph elaborates on that point using concrete examples as evidence.
Does a topic sentence have to be at the beginning of a paragraph?
No, though this is usually the most logical place for it. Sometimes a transitional sentence or two will come before a topic sentence:
We found in comedy that the term bomolochos or buffoon need not be restricted to farce, but could be extended to cover comic characters who are primarily entertainers, with the function of increasing or focusing the comic mood. The corresponding contrasting type is the suppliant, the character, often female, who presents a picture of unmitigated helplessness and destitution. Such a figure is pathetic, and pathos, though it seems a gentler and more relaxed mood than tragedy, is even more terrifying. Its basis is the exclusion of an individual from the group; hence it attacks the deepest fear in ourselves that we possess—a fear much deeper than the relatively cosy and sociable bogey of hell. In the suppliant pity and terror are brought to the highest possible pitch of intensity, and the awful consequences of rejecting the suppliant for all concerned is a central theme of Greek tragedy.
The context for this passage is an extended discussion of the characteristics of tragedy. In this paragraph, Frye begins by drawing a parallel between the figure of the buffoon in comedy and that of the suppliant in tragedy. His discussion of the buffoon occurred in a earlier section of the chapter, a section devoted to comedy. The first sentence of the current paragraph is transitional: it prepares the way for the topic sentence. The delayed topic sentence contributes to the coherence of Frye's discussion by drawing an explicit connection between key ideas in the book. In essays, the connection is usually between the last paragraph and the current one.
Sometimes writers save a topic sentence for the end of a paragraph. You may, for example, occasionally find that giving away your point at the beginning of a paragraph does not allow you to build your argument toward an effective climax.
How do I come up with a topic sentence? And what makes a good one?
Ask yourself what's going on in your paragraph. Why have you chosen to include the information you have? Why is the paragraph important in the context of your argument? What point are you trying to make?
Relating your topic sentences to your thesis can help strengthen the coherence of your essay. If you include a thesis statement in your introduction, then think of incorporating a keyword from that statement into the topic sentence. But you need not be overly explicit when you echo the thesis statement. Better to be subtle rather than heavy-handed. Do not forget that your topic sentence should do more than just establish a connection between your paragraph and your thesis. Use a topic sentence to show how your paragraph contributes to the development of your argument by moving it that one extra step forward. If your topic sentence merely restates your thesis, then either your paragraph is redundant or your topic sentence needs to be reformulated. If several of your topic sentences restate your thesis, even if they do so in different words, then your essay is probably repetitive.
Does every paragraph need one?
No, but most do. Sometimes a paragraph helps to develop the same point as in the previous paragraph, and so a new topic sentence would be redundant. And sometimes the evidence in your paragraph makes your point so effectively that your topic sentence can remain implicit. But if you are in doubt, it's best to use one.