The term “three-period lesson” can refer to two aspects of Montessori pedagogy. Directly, it refers to the basic structure of a Montessori lesson, with each period corresponding to a section of the presentation. In short, the three periods are often illustrated as follows: “This is…” (first period), “Show me…” (second period), and “What is…?” (third period). A Lower Elementary teacher, for example, in giving a lesson on types of triangles, may present three different wooden triangles: a scalene, an isosceles, and an equilateral triangle. The first period of the lesson would consist of the giving of information and nomenclature. “This is a scalene triangle. All of its sides are of different lengths. This is an isosceles triangle. Two of its sides are the same, and one is different. This is an equilateral triangle; all of its sides are equal.” The second period gives the child a reference point. “Show me the equilateral triangle. Show me the scalene triangle. Show me the triangle with three different sides. Show me the triangle whose name means ‘same sides.’” The third period of the lesson removes that reference. “What is this? Which is this one?”

We can take the concept of the three-period lesson a bit further. We can identify experiences and activities that are giving information as the first period, activities that allow the children to work with the concept as the second period, and the presentation of work as the third period. In traditional classrooms, emphasis is placed on the first and third periods. “Here are the names and dates. In two weeks you will have a test and be asked to give back these names and dates.” Seen in this expanded view, we can see that the vast majority of work in a Montessori classroom is much more meaningful second-period work, such as the activities of children working with the materials, finding similar concepts in the environment, making small booklets, creating timelines, or determining the areas of rugs in the classroom. The child ultimately arrives at “third-period” comprehension, but it is a more profound, internalized understanding.