Topic Sentences
When we conclude the reading of one paragraph and move into a new paragraph, we wonder
to ourselves, “What is the connection between this paragraph and the previous paragraph?”
For example, we will want to know if the new paragraph will continue along in the same line of
thought, adding to the idea expressed in the previous paragraph; or will the new paragraph
change direction, contrasting with what came before. The topic sentence must make this
connection clear for the reader.
Take this topic sentence, for example:

However, the glory days of collecting ping-pong paddles will soon come to an end.
Note how this topic sentence points both forward and backward. As a topic sentence, the
sentence points forward by preparing us for the paragraph’s main idea—that the age of
collecting ping-pong paddles is soon coming to an end.
But this topic sentence points backwards as well. It does this through the simple use of the
transitional word “however.” From that one word we can guess the content of the previous
paragraph even without seeing it. We will assume that the previous paragraph emphasizes only
the positive aspects of collecting ping-pong paddles. Thus the movement from the benefits of
paddle collecting into a paragraph discussing a serious collection-related problem entails a
definite change of direction—and the transitional word “however” signals this change of
direction.
Some topic sentences are self-sufficient, allowing readers to clearly see how the new paragraph
connects with the previous paragraph. Other topic sentences need a little help, which is
supplied through the addition of transitional words and phrases. The same transitions that
work to connect sentences can be used to connect paragraphs. (See the “Transitions” section
contained in this Writing Guide.)
In summary, topic sentences perform two functions:
1. They point forward, defining the content of the upcoming paragraph. (This is the topic
sentence function.)
2. They point backwards, connecting the new paragraph to the previous paragraph. (This is
the transitional function, which may be created with transitional words or may be
simply implied.)