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Sides and Corners of 2D Shapes

This lesson teaches students how to identify and count the sides and corners of common 2D shapes, including triangles, squares, rectangles, pentagons, and hexagons, building a foundation for recognizing and naming polygons.

What Are Sides and Corners?

Every flat shape you draw on paper is built from two simple parts: sides and corners. A side is a straight line that forms part of the outline of a shape. A corner, sometimes called a vertex, is the point where two sides meet. If you trace your finger around the edge of a shape, every time your finger changes direction, you have found a corner.

Learning to spot sides and corners is one of the first steps in getting to know 2D shapes. If you have not looked at flat shapes yet, it helps to start with an introduction to 2D shapes before counting their parts here.

Counting Sides and Corners in Common Shapes

Let's look at a few familiar shapes and count their sides and corners one at a time.

Red dots mark each corner; the straight blue lines between them are the sides.

Notice a pattern in these examples: a triangle has 3 straight sides and 3 corners, a square has 4 sides and 4 corners, a pentagon has 5 sides and 5 corners, and a hexagon has 6 sides and 6 corners.

Why Sides and Corners Always Match

For flat shapes with straight sides, the number of corners is always the same as the number of sides. This makes sense because every side needs two endpoints, and each endpoint is shared with the next side, forming a corner. This rule is a big part of how we sort polygons and non-polygons, since a true polygon must be made only of straight sides that close up without any gaps.

Shapes with curves, like circles and ovals, break this pattern. A circle has no straight sides and no corners at all, which is one reason it is not considered a polygon.

Worked Example: Counting a Hexagon's Parts

Look at a hexagon and count carefully by moving around the shape one corner at a time.

Start at any corner and place your finger there. Count "1".
Move along the first straight side to the next corner. Count "2".
Keep moving along each side to the next corner, counting each one: "3", "4", "5", "6".
When you land back on your starting corner, stop counting.

Since you counted 6 corners while moving along 6 straight sides, the shape has 6 sides and 6 corners, which confirms it is a hexagon.

Sides and Corners in 3D Shapes

Flat 2D shapes only have sides and corners because everything lies on one flat surface. Solid shapes are different. A box or a pyramid has flat surfaces called faces, straight lines called edges, and points called vertices instead of simple sides and corners. If you want to see how these ideas connect, take a look at comparing 2D and 3D shapes to see how a flat shape's sides relate to a solid shape's faces and edges.

Quick Recap

To find the sides and corners of any 2D shape, trace the outline slowly. Every straight segment you travel along is a side, and every point where the direction changes is a corner. Counting these carefully lets you name a shape correctly and tell it apart from other shapes that might look similar at first glance.

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