Paragraph unity involves two related but distinct concepts: coherence and flow. Coherence means that the ideas fit together. Flow means that the sentences link up so that readers are not conscious of gaps. Flow is a matter of style and exists in specific words and grammatical patterns tying one sentence to another. Coherence belongs to the substructure of the paragraph, to relationships of thought, feeling, and perception. Both necessary if a paragraph is to be truly unified.
To be coherent a paragraph must satisfy two criteria: First, idea must relate to the topic. Second, effective must be arranged in a way that clarifies their logic or their importance. There is, in addition, a negative that nothing vital must be omitted.
A topic sentence makes a promise that the paragraph must fulfill. Do not wander from the topic. No matter how attractive an idea may seem, let it go if you cannot fit it into the topic you have staked out or cannot revise the topic to include it. Here is an example of a paragraph marred by irrelevance:
 College is very different from high school.  The professors talk a great deal more and give longer homework assignments.  This interferes with your social life.  It may even cost you your girlfriend.  Girls don't like to be told that you have to stay home and study when they want to go to a show or go dancing.  So they find some other boy who doesn't have to study all the time.  Another way college is different is the examinations. . . .
The paragraph begins well. The first sentence establishes the topic and the second supports it. Then the writer begins to slide away. Sentences 3 and 4 might be allowed if they were subordinated. But 5 and 6 lose contact. True, some people do not like to take second place to homework, but that is not pertinent here. In sentence 7 the writer tacitly acknowledges that he has wandered, throwing out a long transitional lifeline to haul us back to the topic. Rid of irrelevance, the paragraph might read:
College is very different from high school. The professors talk a great deal more and give longer homework assignments, which interfere with your social life. College examinations, too, are different. . . .
Order of Thought
Relevance alone is not enough to establish coherence. All the ideas in a paragraph can relate to the topic yet be poorly arranged.
Arrangement often inheres in the subject itself. A paragraph about baking a cake or preparing to water-ski is committed to following the steps of the process it describes. Telling a story, you must follow a certain sequence of events. And in some subjects there is a logical structure implicit in the subject that determines order of thought, as in this example about the value of opposition in politics:
The opposition is indispensable. A good statesman, like any other sensible human being, always learns more from his opponents than from his fervent supporters. For his supporters will push him to disaster unless his opponents show him where the dangers are. So if he is wise he will often pray to be delivered from his friends, because they will ruin him. But, though it hurts, he ought also to pray never to be left without opponents; for they keep him on the path of reason and good sense. Walter Lippmann
There is a necessary order of thought here: the assertion, next a reason supporting it, and then a conclusion, introduced by "so."
There are times, however, when the order of thought is less a function of the subject itself than of the writer's view of it. For instance, if you were writing about the three things that most surprised you the first time you visited, say, New York City, you might not find any logical or temporal relationship between those things.
One solution in such cases is to arrange ideas in order of relative importance, either climactically, placing the most important last, or anticlimactically, putting it first. If you cannot discern any shadings of importance, consider which order best connects with what has gone before or with what will come next. Should you find no basis whatever for arranging the ideas within a paragraph, then, of course, any order is legitimate. But this is not likely to happen often. Most of the time a proper or at least a most effective way of sequencing ideas does exist.
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