A line bounding a plane figure. A surface bounding a solid figure.
In geometry, a side is one of the straight edges that form the boundary of a shape.
For two-dimensional (2D) shapes:
- A side is a line segment that forms part of the outline of a polygon.
- For example, a triangle has 3 sides, and a rectangle has 4 sides.
For three-dimensional (3D) shapes:
- A side can refer to one of the flat faces that forms the boundary of the solid.
- For example, a cube has 6 square sides (also called faces).
We use sides to:
- Classify shapes (like triangles by their side lengths)
- Measure perimeter
- Calculate area and surface area
When Do Students Learn About Sides?
Students begin learning about sides as they explore shapes in elementary school.
Grades K–2 – Recognizing Sides in Shapes
Students count and identify sides of basic shapes like triangles, squares, and rectangles.
Grades 3–5 – Measuring and Comparing Sides
Students measure side lengths, calculate perimeter, and classify shapes based on their sides.
Grades 6+ – Using Sides in Geometry
Students use side lengths to calculate area, surface area, and apply geometric theorems.
