In this section, students take a deeper look at the number line. In elementary school they learned how to place fractions on the number line and in middle school they added negative numbers and learned about the existence of a few isolated irrational numbers, such as $\pi$ and $\sqrt{2}$. Now they see infinite decimal expansions as a way of locating any number, rational or irrational, on the number line. They also expand their repertoire of irrational numbers by considering sums and products of rational with irrational numbers.