The <em>controlling idea</em> in a piece of text tells the reader how the paragraph is held together. It indicates the structure of the paragraph in order to help the reader comprehend the relationship between the various sentences in the paragraph.
Take this statement as an example:
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<em>World War I was a dark event in the annals of human history; millions of people died, and many beautiful landscapes were destroyed.</em>
By analyzing key phrases inside the statement such as dark event, people died, and landscapes were destroyed, one can tell that the controlling idea in the text is the dire consequences of World War I.
