In a previous unit, students learned about standard length units. They created their own rulers by iterating centimeter cubes and used their understanding of length units to measure the length of objects with rulers, meter sticks, and other tools. Students interpreted the scale of line plots and created their own line plots by relating their structure to the length units on measurement tools.

In this lesson, students learn about the number line, a diagram that represents numbers as lengths from 0 using equally spaced tick marks or points. Students learn how to locate whole numbers on the number line and students are introduced to the idea of using a point to represent specific numbers on the number line. Students build on their experiences with measurement to construct a number line to represent numbers within 20. They learn that unlike a linear measurement tool or the line plot they have used, the unit intervals on a number line do not correspond to a specific unit of measure. In the Lesson Synthesis, students consider how number lines with different unit intervals can be used to represent the same numbers and relationships.