Topic sentences have a difficult history in the business of teaching writing. In the 1960s, an influential article was published in a scholarly journal read by writing teachers which proclaimed that research proved that topic sentences were practically nonexistent. However, that "research" was based on light, journalistic, entertainment-oriented texts where topic sentences would be the equivalent of lead boots.

Actually, topic sentences play an important role in giving a document coherence—especially technical, scientific, and business documents. In light, journalistic, entertainment-oriented writing, topic sentences of a subtle, unobtrusive type are used. These are discussed later in this chapter.

The rhetoric and composition scholars may have done some research on the anatomy of the topic sentence and the strategies for its use, but it's doubtful. Not sexy enough. What you'll see here are some efforts to identify several types of topic sentences and some efforts to determine which types fit which writing situations.

Note: There are some differences between topic sentences and thesis sentences, but they are minor. You can read this chapter as applying to thesis sentences as well.

Anatomy of the Topic Sentence

But first what is the topic sentence? It is a sentence occurring toward the beginning of a paragraph that in some way tips the reader off as to the focus, purpose, contents of that paragraph (and perhaps one or more paragraphs following). At its best, it focuses the reader's attention; it says "hey, here's what we're talking about!" Here's an example:

There is no doubt that Gage suffered the accident, and that it had a dramatic impact on his life. However, in his book An Odd Kind of Fame: Stories of Phineas Gage, Australian psychologist Malcolm Macmillan casts serious doubts on the accuracy of the account that entered both scientific and popular discourse. First, very little is known about Gage's personality and habits before the accident. . . .

In this example, serious doubts on the accuracy of the account is the core of the topic sentence, indicating the focus of the following discussion, which is "doubts." Notice it does not begin the paragraph; the first sentence seems to be transitioning from a previous paragraph.

In a scientific, technical, business, or government context where readers are struggling to comprehend, topic sentences seem like help from Above! However, in a light, journalistic, entertainment-oriented context, such a topic sentence might seem like a massive wet blanket. Consider this journalistic example; notice the absence of a topic sentence.

Before the Big Bang

The Catholic Church, which put Galileo under house arrest for daring to say that Earth orbits the sun, isn't known for easily accepting new scientific ideas. So it came as a surprise when Pope Pius XII declared his approval in 1951 of a brand new cosmological theory——the Big Bang. What entranced the pope was the very thing that initially made scientists wary: The theory says the universe had a beginning, and that both time and space leaped out of nothingness. It seemed to confirm the first few sentences of Genesis.

What triggered the Big Bang? According to a new theory, our universe crashed into another three-dimensional world hidden in higher dimensions. "The model suggests a radically different view of cosmic history in which the key events shaping the structure of the universe occurred before the Big Bang," says cosmologist Paul Steinhardt. Eventually, astrophysicists followed the pope's lead, as evidence for the Big Bang became too powerful to ignore. They accepted the notion that the entire observable universe——100 billion galaxies, each stuffed with 100 billion stars, stretching out more than 10 billion light-years in all directions——was once squashed into a space far smaller than a single electron. They bought the idea that the cosmos burst into existence precisely 13.7 billion years ago and has been expanding ever since. But even now, many astrophysicists are still uncomfortable with the implication that the Big Bang marked the beginning of time itself. And the theory has yet to yield a satisfactory answer to a key question: What made the Big Bang go bang?

Michael D. Lemonick, "Before the Big Bang,"
Discover Magazine, February 6, 2006.

It takes a while, but you finally realize the topic is the Big Bang Theory. So now you know the playing field for the topic-sentence game.

Types of Topic Sentences

It could be that writers use a spectrum of topic-sentence types, some of which fill the needs of those happy-go-lucky journalistic writers, while others fill the needs of those struggling writers of scientific, technical, business, or government texts. We don't know because the rhetoric and composition scholarly community has not stooped to consider such issues (unless of course some lonely mouse-like scholar in some hidden tiny college has published something in a little-known journal).

Topic reference. The simplest—and weakest—form of topic sentence is one in which the topic of the paragraph occurs in the sentence. Otherwise, there is no indication as to what will be said about the topic or what the paragraph will cover.

The surface of Mars is thought to be primarily composed of basalt, based upon the Martian meteorite collection and orbital observations. There is some evidence that a portion of the Martian surface might be more silica-rich than typical basalt, perhaps similar to andesitic stones on Earth, though these observations may also be explained by silica glass. Much of the surface is deeply covered by iron oxide dust as fine as talcum powder. There is conclusive evidence that on the surface of Mars liquid water existed at one time. Key discoveries leading to this conclusion include the detection of various minerals such as hematite and goethite which usually only form in the presence of water.

In this example, the topic is surface of Mars but there is no thesis or overview—nor could there be.

Topic definition. A slightly more complex form of topic sentence occurs when the topic sentence defines—or begins the definition of—a term. Call it a topic-definition topic sentence.

In geology, a subduction zone is an area on Earth where two tectonic plates meet and move towards one another, with one sliding underneath the other and moving down into the mantle, at rates typically measured in centimeters per year. An oceanic plate ordinarily slides underneath a continental plate; this often creates a zone with many volcanoes and earthquakes. In a sense, subduction zones are the opposite of divergent boundaries, areas where material rises up from the mantle and plates are moving apart.

In this example, the definition of the topic subduction zone is begun in the first sentence and is continued in the rest of the paragraph.

Keyword topic sentence. One stronger type of topic sentence contains a keyword that sets up an expectation as to the content and organization of the rest of the paragraph or paragraphs. Common keywords include "reasons," "aspects," "factors," and generally any suspicious-looking plural noun at the beginning of a paragraph. Indicative too are modifying words such as "several," "a number of," and so on.

During Samhain there are a number of activities the Celts took part in that resemble some customs we observe on Halloween today. One such custom is trick-or-treating, which may have arisen from the bonfires that the Celts had at this time because children would go door to door asking for firewood. Along with wood, Celts may have collected food door to door for an offering to honor their gods. In honor of these gods, the people may have dressed up to look like them indicating a possible start of wearing costumes. Other costume related events were when the Celts would dress up in ghoulish outfits to parade through the streets and eventually out of town to lead the wandering spirits away from the town.

In this example, the keywords number of activities indicate that the paragraph (and possibly additional paragraphs following) will discuss several Celtic activities that resemble modern Halloween activities. True, this topic sentence could also be seen as a thesis-statement topic sentence, to be discussed in the following. The key difference, however, is the expectation that this keyword type sets up.

Overview topic sentence. Some paragraphs or groups of paragraphs discuss multiple subtopics. An overview topic sentence lists those subtopics.

Most brains exhibit a visible distinction between gray matter and white matter. The gray matter makes up the outer layer of the brain, called cerebral cortex. Gray matter consists of the cell bodies of the neurons, while white matter consists of the fibers (axons) that connect neurons. The axons are surrounded by a fatty insulating sheath called myelin, giving the white matter its distinctive color. Deep in the brain, compartments of white matter (fasciculi, fiber tracts), gray matter (nuclei) and spaces filled with cerebrospinal fluid (ventricles) are found.

In this example, the topic sentence indicates that the two subtopics, gray matter and white matter, will be discussed.

Thesis-statement topic sentence. Some topic sentences make an assertion—an argument—that the rest of paragraph must support.

Although Babbage's machines were mechanical monsters, their basic architecture was astonishingly similar to a modern computer. The data and program memory were separated, operation was instruction based, control unit could make conditional jumps, and the machines had separate I/O units.

In this example, the topic sentence states a thesis: that Babbage's calculating machine is "astonishingly similar" to the modern computer. You expect the rest of the paragraph to support that thesis.

Suspense-oriented nontopic sentence. Some writers enjoy messing with your readerly brains. They intentionally hide the logic connecting the current paragraph to anything preceding. Charlatans!

We leave traces of ourselves wherever we go, on whatever we touch. One of the odd discoveries made by small boys is that when two pebbbles are struck sharply against each other they emit, briefly, a curious smoky odor. The phenomenon fades when the stones are immaculately cleaned, vanishes when they are heated to furnace temperature, and reappears when they are simply touched by the hand again before being struck.

In this example, from the beginning of Lewis Thomas's "Vibes" appearing in his The Lives of a Cell, we're not sure where Lewis Thomas is headed with this discussion. After another paragraph or two, you'll see that the topic is odors. The idea is to create a sense of wonder and suspense.

Revising with Topic Sentences

It's not an instinctive matter to start a paragraph or section with a topic sentence. Few of us have that kind of foresight. Instead, topic sentences are likely to be added, in those paragraphs where they are really needed, in the revision phase of a writing project. If writing is often a process of discovery, we discover the need for topic sentences later in that process.

Revision to include a topic sentence.

Choosing Topic Sentences

Your best guide for deciding when to use topic sentences and which type is probably your own instincts and intuition. But here are some ideas: