from: http://grammar.ccc.commnet.edu/
Irrelevant Details
When I was growing up, one of the places I enjoyed most was the cherry tree in the back yard. Behind the yard was an alley and then more houses. Every summer when the cherries began to ripen, I used to spend hours high in the tree, picking and eating the sweet, sun-warmed cherries. My mother always worried about my falling out of the tree, but I never did. But I had some competition for the cherries — flocks of birds that enjoyed them as much as I did and would perch all over the tree, devouring the fruit whenever I wasn't there. I used to wonder why the grown-ups never ate any of the cherries; but actually when the birds and I had finished, there weren't many left.
No sentence is completely irrelevant to the general topic of this paragraph (the cherry tree), but the sentences Behind the yard was an alley and then more houses and My mother always worried about my falling out of the tree, but I never did do not develop the specific idea in the first sentence: enjoyment of the cherry tree.
Shift in Focus
1It is a fact that capital punishment is not a deterrent to crime. 2Statistics show that in states with capital punishment, murder rates are the same or almost the same as in states without capital punishment. 3It is also true that it is more expensive to put a person on death row than in life imprisonment because of the costs of maximum security. 4Unfortunately, capital punishment has been used unjustly. 5Statistics show that every execution is of a man and that nine out of ten are black. 6So prejudice shows right through.
Once again, no sentence in this paragraph (to the left) is completely irrelevant to the general topic (capital punishment), but the specific focus of this paragraph shifts abruptly twice. The paragraph starts out with a clear claim in sentence 1: It is a fact that capital punishment is not a deterrent to crime. Sentence 2 provides evidence in support of the initial claim: Statistics show that in states with capital punishment, murder rates are the same or almost the same as in states without capital punishment. Sentence 3, however, shifts the focus from capital punishment as a deterrent to crime to the cost of incarceration: It is also true that it is more expensive to put a person on death row than in life imprisonment because of the costs of maximum security. Sentence 4 once again shifts the focus, this time to issues of justice: Unfortunately, capital punishment has been used unjustly. Sentences 5 and 6, Statistics show that every execution is of a man and that nine out of ten are black and So prejudice shows right through, follow from 4 if one believes that executing men and blacks is in fact evidence of injustice and prejudice. More importantly, however, we are now a long way off from the original claim, that capital punishment does not deter crime. The focus has shifted from deterrence to expense to fairness.
Topic Sentences
All three paragraphs start out well with a topic sentence. A topic sentence is a sentence whose main idea or claim controls the rest of the paragraph; the body of a paragraph explains, develops or supports with evidence the topic sentence's main idea or claim. The topic sentence is usually the first sentence of a paragraph, but not necessarily. It may come, for example, after a transition sentence; it may even come at the end of a paragraph.
Topic sentences are not the only way to organize a paragraph, and not all paragraphs need a topic sentence. For example, paragraphs that describe, narrate, or detail the steps in an experiment do not usually need topic sentences. Topic sentences are useful, however, in paragraphs that analyze and argue. Topic sentences are particularly useful for writers who have difficulty developing focused, unified paragraphs (i.e., writers who tend to sprawl). Topic sentences help these writers develop a main idea or claim for their paragraphs, and, perhaps most importantly, they help these writers stay focused and keep paragraphs manageable.
Topic sentences are also useful to readers because they guide them through sometimes complex arguments. Many well-known, experienced writers effectively use topic sentences to bridge between paragraphs. Here's an example of how one professional writer does this:
Soon after the spraying had ended there were unmistakable signs that all was not well. Within two days dead and dying fish, including many young salmon, were found along the banks of the stream. Brook trout also appeared among the dead fish, and along the roads and in the woods birds were dying. All the life of the stream was stilled. Before the spraying there had been a rich assortment of the water life that forms the food of salmon and trout — caddis fly larvae, living in loosely fitting protective cases of leaves, stems or gravel cemented together with saliva, stonefly nymphs clinging to rocks in the swirling currents, and the wormlike larvae of blackflies edging the stones under riffles or where the stream spills over steeply slanting rocks. But now the stream insects were dead, killed by DDT, and there was nothing for a young salmon to eat.
Rachel Carson, Silent Spring
The first part of Carson's topic sentence — Soon after the spraying had ended — is a transitional clause that looks back to the previous topic: DDT spraying. Topic sentences often begin with such transitional clauses referring to the previous paragraph. The second part of the topic sentence — there were unmistakable signs that all was not well — shapes and controls what follows. This kind of bridging helps the reader follow Carson's argument. Notice, too, how Carson further helps the reader follow her argument by providing a more focused version of the topic sentence later in the paragraph — All the life of the stream was stilled. This sentence tells us exactly what Carson meant by all was not well.