In this lesson, students examine and make use of the structure of the number line (MP7). Students begin by considering which values could be represented by a point on a horizontal number line by reasoning about the position in relation to other known values on the number line. Then they revisit the context of temperature, represented on a vertical number line, extending previous work interpreting equally spaced divisions to the negative part of the number line.

Next, students create folded number lines out of tracing paper to reinforce the symmetry of the number line and to reason about opposites—numbers that are the same distance from zero but on opposite sides of the number line.

Lastly, students are introduced to the term rational number as a number that can be expressed as a positive or negative fraction or zero. Students will have more practice placing rational numbers on the number line in future lessons. The focus in this lesson is the concept of opposites rather than plotting different kinds of rational numbers.