Lesson objective: Find the perimeter of a polygon marked with unit length markers, but without numbers.

This lesson helps to build procedural skill with determining perimeter. Polygon models are used here because they provide a visual that supports the skill of finding perimeter. This work develops students' understanding that perimeter is the sum of the length of the sides of a polygon.

Students engage in Mathematical Practice 5 (Use appropropriate tools strategically) as they use unit length markers to determine perimeter in the same way they would use a ruler. They can also use other length measuring tools but without numbers marking the units.

**Key vocabulary:**

- perimeter
- polygon
- sides
- unit