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What is a Paragraph?
There are two ways to examine a paragraph. One is visual; the other is rhetorical, that is, what a paragraph does.
I am sure you can recognize a paragraph by looking at a page of text. In American English, each paragraph is indented, usually one tab or 5 to 7 spaces. The indentation gives the reader a little "breathing room." It allows the reader to take a little break. When you come to the end of a paragraph, it may stop midline and not continue all the way to the margin.
Ok, so that's the visual. But what is a paragraph with regard to writing? Many English handbooks define a paragraph as "a group of sentences that present and develop one main idea."
Usually, but not always, a paragraph begins with a Topic Sentence, the sentence that expresses the controlling idea of the paragraph. It mentions the subject, what the paragraph is about, and the treatment, what the writer will say about the subject.
Well developed paragraphs also contain supporting sentences, sentences that provide evidence such as details, examples, and explanations that explain the topic sentence.
Sentences that do NOT support the topic sentence are "off-topic" and do NOT belong in the paragraph. Sometimes they just need to be deleted entirely; other times they can be moved to a different paragraph where they DO belong. You can recognize off-topic sentences if they do not relate to the topic sentence in some way. This principle regards the idea of UNITY in writing.