The topic sentence in each paragraph tells about the limited subject matter to be discussed in that paragraph. Usually, but not always, the topic sentence is the first sentence in the paragraph.
Anyone who travels in Ontario will be struck by the number of towns and cities that have famous namesakes in Britain and Europe.
The controlling idea is the focus of the topic sentence: it is the crux of the sentence, the idea about which the paragraph is to be written. In the sample sentence above, "famous namesakes" is the controlling idea. The chief concern in writing this paragraph would be to mention cities like Windsor ON, London ON, Paris ON, Cornwall ON, which have famous namesakes in Britain and France. Any Windsor, London, Paris, Cornwall not located in Ontario could not be included in the discussion because these cities do not fit the criteria of the controlling idea (Ontario towns and cities that have famous namesakes