The unit of scholarly prose is the paragraph and the key to writing a good paragraph is to support, elaborate or defend a single, well-defined claim. This claim should be stated plainly in one of the sentences of the paragraph, which we call the “key sentence”. It should stand out in some way from the other sentences in the paragraph, often by being shorter and simpler. There is no rule about where in the paragraph it should appear. It is often the first sentence, sometimes the last. But it can, in fact, go anywhere. The important thing is that the reader comes to experience it as the focus of the paragraph.
Remember that it takes about a minute to read a paragraph. During that minute you are shaping your reader’s attention, guiding it in a certain direction. If you leave the key sentence til the end, it should seem to arrive naturally in a space that had been arranged for it. If you put it at the beginning, it should be borne out by the rest. If you put it in the middle, there should be a feeling of being led towards it and then away from it. But whatever you do, the key sentence itself should declare something to be true. It should tell the reader what you want them to believe after reading the paragraph. They may not believe you, of course; but it should be clear what they would have believed if they did.
And it should be something that you believe–and not just believe, in fact, but know. It should state a justified, true belief about which you able to converse intelligently. It should appear true to you, and you should know why it is true. That means it’s not just a heading or theme. It’s a sentence. It can be true or false.
The key sentence is the focus of your writing moment.
Composing your paragraphs around key sentences also helps you to organize your texts as a whole. Think of it as a series of claims, each supported, elaborated or defended in a paragraph, and think of your key sentence as stating that claim without support, elaboration or defense. That means that your text can be summarized in a key sentence outline, a simple list of your key sentences. While they should fail to persuade or enlighten on their own (that’s what their paragraphs are for), they should indicate a line of argument. They should make sense even without the prose in which they are embedded. If your key claims make sense on their own, simply stated as a series of assertions, and if each of your paragraphs provides the support, elaboration or defense that is appropriate, then you don’t have to worry much more about the structure or “flow” of your paper. You can of course still think about paragraph transitions and clever allusions if you wish, but these are finishing touches. The substance will already be nicely in place.
Key sentences, then, really are the keys to good paragraphs and to good compositions.