Sentences that do not support the topic sentence destroy the unity of a paragraph. If the paragraph is otherwise well focused, such offending 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 in color) is clearly off the point.
PARAGRAPH WITH DETAIL THAT IS 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. You might begin straying from the main point of the paragraph because you cannot think of enough evidence to support the topic sentence. When this happens, you are faced with a choice: Either find more evidence to support the topic sentence or rewrite the topic sentence to reflect the evidence that is available.
Exercise: Topic sentences
Related topics:
Stating the main point in a topic sentence
Developing the main point