The main point in classification uses a single organizing principle to sort items in a way that serves your purpose. The categories should help you achieve your purpose. The main point may or may not state the organizing principle directly. Look at the following examples:

TOPIC SENTENCE:

THESIS STATEMENT:

In both main points, the organizing principle is types of things — columns in the case of the paragraph and buildings in the case of the essay. The thesis statement does not state this principle directly, however. Instead, the categories — stadiums, theaters, and temples — make the principle clear.

Also, notice that the topic for the essay is broader than the one for the paragraph, which is often the case.