“A paragraph is unified when every sentence develops the point made in the topic sentence. It must have a single focus and it must contain no irrelevant facts. Every sentence must contribute to the paragraph by explaining, exemplifying, or expanding the topic sentence. In order to determine whether a paragraph is well developed or not, ask yourself: ‘What main point am I trying to convey here?’ (topic sentence) and then ‘Does every sentence clearly relate to this idea?”
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It is important that you understand what a topic sentence is in order to write a unified and coherent paragraph. A topic sentence (also known as a focus sentence) encapsulates or organises an entire paragraph, and you should be careful to include one in most of your major paragraphs. Although topic sentences may appear anywhere in a paragraph, in academic essays they often appear at the beginning. If you are still unsure about topic sentences go to this excellent site.