We are going to first introduce the concept of nested control structures. Nesting is a concept that places one item inside of another. Consider:
if expression true action else false action
This is the basic form of the if then else control structure. Now consider:
if age is less than l8 you can't vote if age is less than l6 you can't drive else you can drive else you can vote if age is less than 2l you can't drink else you can drink
As you can see we simply included as part of the "true action" a statement and another if then else control structure. We did the same (nested another if then else) for the "false action". In our example we nested if then else control structures. Nesting could have an if then else within a while loop. Thus, the concept of nesting allows the mixing of the different categories of control structures.