The '''contrapositive''' of a statement of the form "A implies B" is the [[logic]]ally equivalent statement "not-B implies not-A."
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Do not confuse the '''contrapositive''' with the [[converse]], which is very different and not necessarily true.
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For example, let A be the statement "R is a square" and B be the statement "R is a rectangle". Then A implies B. The contrapositive of this is the statement "if R is not a rectangle, then R is not a square", which is logically equivalent to the first statement. On the other hand, the converse is "if R is a rectangle, then R is a square", an implication which isn't even true!