Identify & name polygons
In Year 3 (NSW Stage 2, outcome MA2-2DS-01), students identify and name polygons: closed 2D shapes made only of straight sides. A polygon is named by its number of sides, so a triangle has 3, a quadrilateral 4, a pentagon 5, a hexagon 6 and an octagon 8.
Students count sides and vertices (a polygon always has the same number of each), name shapes that are tilted or have unequal sides, and explain why circles and open shapes are not polygons. They also meet regular polygons, such as the octagon on a stop sign, where every side and every angle is the same.
Theory
Identifying and naming polygons is a Year 3 space skill: a polygon is a closed 2D shape made only of straight sides. This page shows how to name a polygon by counting its sides — triangle, quadrilateral, pentagon, hexagon and octagon — how to tell polygons from shapes like circles, and what makes a polygon regular or irregular.
A polygon is a flat (2D) shape that is closed and made only of straight sides. Closed means the sides join up all the way around with no gaps. A shape with a curved edge, or a shape left open, is not a polygon.
Each straight edge is a side. Each point where two sides meet is a corner (also called a vertex). A polygon always has the same number of corners as sides.
We name a polygon by its number of sides. Three sides make a triangle, four sides make a quadrilateral, five sides make a pentagon, six sides make a hexagon and eight sides make an octagon. A regular polygon has all sides the same length and all corners (angles) the same size; an irregular polygon does not.
To name a polygon, count its straight sides and match the count to the name. Keep this table handy.
| Number of sides | Polygon name |
|---|---|
| 3 | Triangle |
| 4 | Quadrilateral |
| 5 | Pentagon |
| 6 | Hexagon |
| 8 | Octagon |
How to identify and name a polygon
- Check it is a polygon. Make sure the shape is closed and every side is straight. If there is a curve or a gap, it is not a polygon.
- Count the sides. Start at one corner and count each straight side once, going all the way around.
- Match the name. Use the side count to name it: 3 triangle, 4 quadrilateral, 5 pentagon, 6 hexagon, 8 octagon.
- Regular or not? If every side is the same length and every corner the same size, add the word regular.
Start at one corner and count each straight side once, all the way around: there are \(6\) sides (and \(6\) corners).
A polygon with \(6\) sides is a hexagon, even though it looks like the letter L.
A polygon must have only straight sides. This shape has a curved side along the bottom, so it breaks the rule.
It is not a polygon.
A regular polygon has all sides equal and all corners the same size. Here the sides are 2 cm, 2 cm, 5 cm and 5 cm, so they are not all the same.
It is an irregular quadrilateral.
\(3\) straight sides \(\to\) a triangle.
All the sides are equal (and so are the corners), so it is a regular triangle.
Common pitfalls
Frequently asked questions
What is a polygon?
A polygon is a closed 2D shape made only of straight sides. The sides join up with no gaps and no curves. A triangle, a square and a hexagon are all polygons.
How do you name a polygon?
You name a polygon by counting its sides. \(3\) sides is a triangle, \(4\) sides is a quadrilateral, \(5\) sides is a pentagon, \(6\) sides is a hexagon and \(8\) sides is an octagon.
Is a semicircle a polygon?
No. A semicircle has one straight side, but its other edge is curved. A polygon must be made only of straight sides.
What is the difference between a regular and irregular polygon?
A regular polygon has all sides the same length and all corners the same size. An irregular polygon has sides or corners that are not all the same. Both are still polygons.
Can a polygon have only 2 sides?
No. Two straight sides can only meet at one corner and cannot close up a shape. The fewest sides a polygon can have is \(3\), which makes a triangle.
Does a polygon have the same number of sides and corners?
Yes. Every polygon has the same number of corners as sides. A quadrilateral has \(4\) sides and \(4\) corners.