If you want to hold an audience’s attention, paragraph unity and length are important to readability.
Many people write until the text appears to be getting long, then put in a period and hit enter. But a paragraph is not a randomly created chunk of black print. It’s a sequence of related thoughts that explore a topic. When you come to the end and you insert a period and create a paragraph break, you indicate to the reader that you are finished with that topic or that aspect of the discussion and you are moving on to a new fact or to an entirely new topic. Certain qualities define a good paragraph; two of them are unity and length.
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Keeping the paragraph unified
Paragraph unity means that each sentence relates to one main idea, on controlling theme. Veering off and talking about marginally related topics will confuse the reader. One way to unify a paragraph is to open with a topic sentence, which presents the theme. It is not mandatory to have a topic sentence, and often you won’t. So as long as the sentences are linked to one controlling idea, the paragraph is unified. But a succinct opening sentence that frames the paragraph helps guide the reader.
Here are examples of topic sentences:
Oil prices are rising for three reasons. (Then go on to explain them.)
A falling dollar can hit U.S. consumers, investors, and businesses in various ways. (Then talk about the ways.)
Length: How long is too long?
Most paragraphs should be two to four sentences long or five lines of text. So if one sentence consumes two lines, you probably want only two or three sentences in the paragraph. That will put you at about five or six lines of text, which is about as much as the brain can absorb easily. It likes white space.
This is only a guideline; you can use judgment. Some paragraphs will be longer (10 to 20 lines) in a proposal to the EPA, in a piece of academic writing, or in a novel. They will be shorter in an email message. But avoid a series of one-sentence paragraphs.