A well-organized paragraph supports or develops a single controlling idea, which is expressed in a sentence called the topic sentence. A topic sentence has several important functions: it substantiates or supports an essay’s thesis statement; it unifies the content of a paragraph and directs the order of the sentences; and it advises the reader of the subject to be discussed and how the paragraph will discuss it. Readers generally look to the first few sentences in a paragraph to determine the subject and perspective of the paragraph. That’s why it’s often best to put the topic sentence at the very beginning of the paragraph. In some cases, however, it’s more effective to place another sentence before the topic sentence—for example, a sentence linking the current paragraph to the previous one, or one providing background information.
The Oxford English Grammar defines a topic sentence as "[C]ommonly, though not invariably, the first sentence of a paragraph. It . . . conveys a generalization followed by an example" (the supporting details). I should add here that a topic sentence is probably followed by "an example" which, itself, is explained in further detail. (italics added)
So, the topic sentence is general; the rest is detail.
Example: (topic sentence is italicized; remainder of paragraph is detail supporting the topic sentence)
At times, those who govern also regard particular circumstances as too uncomfortable, too painful, for most people to cope with rationally. [the generalization] They may believe, for instance, that their country must prepare for long-term challenges of great importance, such as a war, an epidemic, or a belt-tightening in the face of future shortages. [detail / examples] Yet they may fear that citizens will be able to respond only to short-range dangers. Deception at such times may seem to the government leaders as the only means of attaining the necessary results. [further detail] (from Lying: Moral Choice in Public and Private Life, by Sissela Bok, p. 168.)
Further explanation from The Elements of Style:
Ordinarily, . . .a subject requires division into topics, each of which would be dealt with within a paragraph, The object of treating each topic in a paragraph is, of course, to aid the reader. The beginning of each paragraph is a signal to [the reader] that a new step in the development of the subject has been reached. (italics added)
This is most true when there are several paragraphs regarding a theme. In those cases, the topic sentence is essential to "signal to the reader" that what follows is a different aspect of the theme.
Source: The Oxford English Grammar, by Sidney Greenbaum, 1st Ed.; Lying: Moral Choice In Public and Private Life, by Sissela Bok, p.168; The Elements of Style, by Strunk and White, p.16
Monica Monk explains how to write topic sentences, using a paragraph which contains supporting detail, but is missing its topic sentence. She then writes an appropriate topic sentence for it.
To reiterate: the topic sentence is the main idea, and the rest of the paragraph supports that main idea.
Creating the cohesion needed between the topic sentence and the details isn't easy. You have to try, try again. Write and rewrite. But all that work is the art of writing. And trust me, good writing is an art.
But I digress a bit. The point is the cohesion. To illustrate, let's work backwards, and see the details, the supporting information, first, as see if we can't come up with a decent topic sentence. Here we go:
For example, recently I began taking my two-year-old dog to obedience school. After four weeks of lessons and practice, she has learned to follow only three commands -- sit, stand and lie down -- and even those she often gets confused. Frustrating (and costly) as this is, I continue to work with her every day. After dog school, my grandmother and sometimes go grocery shopping. Inching along those aisles, elbowed by hundreds of fellow customers, backtracking to pick up forgotten items, and standing at the endless line at checkout, I could easily grow frustrated and cranky. But through years of trying times, I have learned to keep my temper in check. Finally, after putting away the groceries, I might go out to a movie with my fiance, to whom I have been engaged for three years. Layoffs, extra jobs, and problems at home have forced us to postpone our wedding date several times. Still, my patience has enabled me to cancel and reschedule our wedding plans again and again without fuss, fights, or tears.
(There are several parts I would rewrite, but let's just stick with the topic sentence for this lesson.)
Okay, there are three examples given of the main idea, about which the writer, let's assume a woman, gives us a hint in the final sentence. She mentions patience, and as we reread the paragraph, we see that the three examples are indeed about her patience. Her examples also mention that she keeps at it, "again and again," so this is also part of the topic.
Let's try this for a topic sentence: "My life is full of things that try my patience, but I have learned to keep at it, to try, try again."
Or this: "Nothing tests my patience more than just going through a normal day."
Maybe this: "I'm telling you, I may not have the patience of Job, but I'm getting there."
We could go on and write the main idea -- the topic sentence -- of patience any number of ways. That's the art of writing.
One more thing. That last sentence, the one that re-mentions patience (brought up originally in the topic sentence), is key to the cohesiveness of this paragraph, in that it brings the three examples back to the main idea. It's the old "tell 'em what you're going to say, say it, then tell 'em what you said."
Source: Practice in Composing Topic Sentences Paragraphs with Examples, by Richard Nordquist, About.com Guide (http://grammar.about.com/od/developingparagraphs/a/practicetopic.htm)
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