Naming 2D Shapes

Counting sides and corners

Look around and you will spot flat shapes everywhere — a square window, a triangular slice of toast, a round biscuit. To talk about them, we give each one a name, and the name comes from a quick bit of counting.

A 2D shape is a flat shape — one you can draw on a piece of paper and colour in. It has no thickness at all, like a shadow. We do not give a flat shape its name by its colour or its size; a tiny red triangle and a huge blue triangle are both triangles. We name it by counting two things:

its sides — the straight edges, the lines you would trace with your finger;
its vertices — the corners, the sharp points where two sides meet. (One corner is a vertex; two or more are vertices.)

Here are the shapes you will meet most often, sorted by how many sides they have:

a triangle has 3 sides and 3 corners;
a quadrilateral has 4 sides — a square and a rectangle are both quadrilaterals;
a pentagon has 5 sides and 5 corners;
a hexagon has 6 sides and 6 corners;
an octagon has 8 sides and 8 corners;
a circle has no straight sides at all — it is one smooth curve with no corners to count.

A flat shape made only of straight sides is called a polygon. So a triangle, a square, a pentagon, a hexagon and an octagon are all polygons — but a circle is not, because part of it is curved.

pizza slice

Once you know the names, you start to spot shapes everywhere. A slice of pizza is a triangle — count its three straight edges. The face of a clock is a circle, with no corners at all. A road sign that says STOP is an octagon, with eight sides. Look around your room right now: a window might be a rectangle, a slice of toast a square, a slice of watermelon a triangle. How many can you find?

Regular shapes and wonky shapes

Two shapes can have the same name even when they look quite different. What matters is the number of sides, not whether the shape is neat.

A regular shape has all its sides the same length and all its corners the same size. A regular quadrilateral is a square; a regular triangle has three equal sides.
An irregular shape — a "wonky" shape — has sides of different lengths, but it keeps its name as long as the number of straight sides is right. A long, thin, lopsided triangle is still a triangle, because it still has exactly three sides.

So a square and a rectangle are both quadrilaterals: each has four sides. The square is the regular one (all four sides equal); the rectangle is a stretched version, but four sides is four sides.

The rule for naming polygons

Notice the handy pattern: for a polygon, the number of corners is always the same as the number of sides. A triangle has 3 of each, a pentagon 5 of each. So once you can count the sides, you already know the name.

for any polygon, the number of sides equals the number of vertices (corners);
counting sides gives the name: triangle 3, quadrilateral 4, pentagon 5, hexagon 6, heptagon 7, octagon 8;
a regular shape has all its sides and all its angles equal (a regular quadrilateral is a square);
a circle is a curve, not a polygon — it has no straight sides and no corners.

Two traps when you are naming a shape:

To find the name, count the sides, not just the points. For these shapes the number of sides and corners is the same, so either works — but get into the habit of counting sides, because that is the rule that always holds.
A shape does not have to be neat to keep its name. A "wonky" five-sided shape with no two sides the same length is still a pentagon — it has five straight sides, and that is all the name asks for.

Three worked examples

A flag is shaped with three straight sides. Three sides means three corners, so it is a triangle — even though it is long and pointy, not the neat kind.
A door is a tall shape with four straight sides: two long ones and two short ones, with square corners. Four sides means it is a quadrilateral — this special one, with square corners, is a rectangle.
A football is stitched from five-sided patches and six-sided patches. Trace one patch with five straight sides: five sides, five corners — a pentagon, however wonky it looks on the curved ball.

coin sun ball

A coin, the Sun and a bouncy ball all look round from the front — they are circles. A circle is the odd one out, because it has no straight sides and no corners — it is a single smooth curve, the same distance round from the middle the whole way. That is exactly why wheels are round: with no corners to bump over, they roll along perfectly smoothly. Try rolling a square and you will see why!

The shape gallery

Step through the gallery to meet one shape at a time. Each is labelled with its name and how many sides it has — count the corners and you will get the same number, except for the circle.

Name the mystery shape