Paragraphs have no specific length and no required number of sentences, but they have three characteristics: they are unified, coherent, and adequately developed. That is, a paragraph presents thought, all its parts are clearly related to one another, and its point is sufficiently supported by details, examples, or explanations.
Each paragraph in a well-written essay supports the thesis sentence; each is directly linked to the thesis and is clearly related to other paragraphs in the essay. The introductory paragraph presents the main point, and the concluding paragraph reinforces it. Paragraphs in the body develop the idea in such a way as to make the essay interesting and convincing. Carefully linked paragraphs provide coherence to an essay, and if each paragraph in the body supports and develops the single main idea of the entire paper, the essay has unity.
The paragraph in coherent and unified essays may be developed by different patterns: by definitions, details, examples, analogy, cause-and-effect analysis, and reason.
A paragraph is unified when all its parts relate clearly to its central, controlling idea which is expressed in a topic sentence.
Where to place the topic sentence? In most common paragraph shape, the central idea expressed in a topic sentence – falls first, sometimes followed by a clarifying or limiting sentence that narrows the topic and makes it more specific.
The central idea may also appear at the end of a paragraph or in its middle. The central idea may appear at the beginning of the paragraph and then be restated or added at the end. It may also be amplified in the middle.
Finally, the central idea may be unstated. The main idea of the paragraph may become obvious from the context. Students should be able to sum up in writing the unstated topic sentence even if they see that it is out of the paragraph.