A paragraph should be unified around a main point. The main point should be clear to readers, and every sentence in the paragraph should relate to it.
Stating the main point in a topic sentence
As readers move into a paragraph, they need to know where they are—in relation to the whole essay—and what to expect in the sentences to come. A good topic sentence, a one-sentence summary of the paragraph’s main point, acts as a signpost pointing in two directions: backward toward the thesis of the essay and forward toward the body of the paragraph. Usually the topic sentence (highlighted in the following example) comes first in the paragraph.
All living creatures manage some form of communication. The dance patterns of bees in their hive help to point the way to distant flower fields or announce successful foraging. Male stickleback fish regularly swim upside-down to indicate outrage in a courtship contest. Male deer and lemurs mark territorial ownership by rubbing their own body secretions on boundary stones or trees. Everyone has seen a frightened dog put his tail between his legs and run in panic. We, too, use gestures, expressions, postures, and movement to give our words point.
—Olivia Vlahos, Human Beginnings
In college writing, topic sentences are often necessary for advancing or clarifying the lines of an argument or reporting the research in a field. In business writing, topic sentences (along with headings) are essential because readers often scan for information and summary statements. Sometimes the topic sentence is introduced by a transitional sentence linking the paragraph to earlier material, and occasionally the topic sentence is withheld until the end of the paragraph.
Sticking to the point
Sentences that do not support the topic sentence destroy the unity of a paragraph. If the paragraph is otherwise focused, such sentences can simply be deleted or perhaps moved elsewhere. In the following paragraph describing the inadequate facilities in a high school, the information about the chemistry instructor (highlighted) is clearly off the point.
As the result of tax cuts, the educational facilities of Lincoln High School have reached an all-time low. Some of the books date back to 1990 and have long since shed their covers. The few computers in working order must share one printer. The lack of lab equipment makes it necessary for four or five students to work at one table, with most watching rather than performing experiments. Also, the chemistry instructor left to have a baby at the beginning of the semester, and most of the students don’t like the substitute. As for the furniture, many of the upright chairs have become recliners, and the desk legs are so unbalanced that they play seesaw on the floor.
Sometimes the solution for a disunified paragraph is not as simple as deleting or moving material. Writers often wander into uncharted territory because they cannot think of enough evidence to support a topic sentence. Feeling that it is too soon to break into a new paragraph, they move on to new ideas for which they have not prepared the reader. When this happens, the writer is faced with a choice: Either find more evidence to support the topic sentence or adjust the topic sentence to mesh with the evidence that is available.