The concept of a contrapositive is easily described with the following example. Suppose we have an if-then statement:
If condition A is true, then condition B is true.
The contrapositive of the above statement is
If condition B is not true, then condition A is not true.
We create the contrapositive of a given statement by swapping conditions A and B, and making each condition false.
If you find that this is a bit abstract, not to worry: we next consider some examples that explain these concepts in a more concrete way.