Students are introduced to perimeter in Topic C. Conceptual exploration begins by creating tessellations. In Lessons 10 and 11, students decompose a quadrilateral. They rearrange the parts to form a new shape. They then use the new shape to tile, tracing its perimeter until a new larger shape (the complete tessellation) is formed. Through this work, students define perimeter as the boundary of a two-dimensional shape and use their new vocabulary in context as they describe the process of tessellating. This lesson begins the study of perimeter with unusual shapes to encourage flexible thinking about perimeter and avoid the misconception that it is a property of rectangles alone. By Lesson 17 students will be using all four operations to determine a perimeter and any missing measurements. Students develop strategies for finding part of a larger shape, for example, the blue rectangle in the figure below. In this example, students understand that they can subtract the known part of the length from the total length to find the missing measurement. The missing measurements may then be used to find the perimeter of the blue rectangle.