The names of 2D shapes

A 2D shape is a flat shape, like one drawn on paper. Most of the shapes children learn have straight sides, and their names say how many: a pentagon has 5 sides, a hexagon 6 and an octagon 8. So you can name a shape by counting, even one you have never seen drawn that way before.

The rule

A flat shape with straight sides is named by how many sides it has: 3 is a triangle, 4 a quadrilateral, 5 a pentagon, 6 a hexagon, 7 a heptagon and 8 an octagon. A square and a rectangle are special quadrilaterals. A circle and an oval have no straight sides at all.

How to name a shape

Look at the edge. If it is one smooth curve, the shape is a circle or an oval. A circle is equally wide whichever way you measure it. An oval is stretched, longer one way than the other.
If the sides are straight, put a finger on one corner and count the sides as you go round, until you are back where you started.
Match the number to the name: 3 sides is a triangle, 4 a quadrilateral, 5 a pentagon, 6 a hexagon, 7 a heptagon and 8 an octagon.
With 4 sides, look closer. If all four corners are square corners, called right angles, it is a rectangle. If the four sides are also all the same length, it is a square.
The size of a shape and the way it is turned never change its name. Nor do uneven sides: any shape with 5 straight sides is a pentagon, even if no two of its sides match.

Worked examples

What are these shapes called?

Count the sides of each one, as the numbers under them do. 3 sides is a triangle. 4 equal sides with square corners is a square. 5 is a pentagon, 6 a hexagon and 8 an octagon, the shape of a stop sign.

What shape is this?

It has 6 sides, so it is a hexagon. Its sides are all different lengths and it looks nothing like a honeycomb cell, but the name only counts the sides.

Are all three of these triangles?

Yes. Each one has 3 straight sides and 3 corners. The first has three equal sides, the second has a square corner, and the third is long and tipped over, but a triangle is any shape with three straight sides.

Which one is the square, and which is the rectangle?

Both have 4 sides and 4 square corners, shown by the little squares. Matching dashes mean matching lengths. On the left, all four sides have one dash, so they are all the same length: a square. On the right, the long sides match and the short sides match, but long and short are different, so it is a rectangle.

More shapes with four sides

Any shape with 4 straight sides is a quadrilateral, and squares and rectangles are only two of them. A rhombus has 4 equal sides, like a square pushed over, so its corners are not square. A parallelogram is a rectangle pushed over in the same way, with its opposite sides still parallel. A kite has two short sides next to each other and two long sides next to each other.

A quadrilateral with just one pair of parallel sides is called a trapezium in British schools and a trapezoid in American ones. Use the word your child's school uses.

Why the names count the sides

Most shape names are old Greek number words joined to gon, which means corner. Penta is 5, so a pentagon has 5 corners. Hexa is 6, hepta is 7 and octa is 8, the same as the octo in octopus, which has 8 arms. Polygon, the name for any flat shape with straight sides, means many corners.

Triangle works the same way in Latin: tri is 3, so a triangle has three angles, three corners. Quadrilateral is Latin too, and means four sides.

A shape always has the same number of corners as sides, because each side runs from one corner to the next. So a name that counts corners counts the sides as well. Counting sides and corners shows how to count them without losing your place.

Where mistakes happen

Naming a shape by how it looks instead of counting. A hexagon with uneven sides is still a hexagon, and a child who only knows the honeycomb shape may not recognise it.
Mixing up pentagon, hexagon and octagon. 5, 6 and 8 sides look much alike at a glance, so count them. Hexagon and six both have an x, and octagon starts like octopus.
Calling a square a diamond when it is turned onto one corner. It is still a square. Turning a shape does not change its sides or its corners.
Calling a rectangle a square because its corners are square. A square needs all four sides the same length as well.
Calling an oval a circle. A circle is the same width across whichever way you measure it. An oval is longer one way than the other.

Questions people ask

Is a square a rectangle?

Yes. A rectangle is a four-sided shape with four square corners, and a square has those, with all four sides equal as well. So every square is a rectangle, but not every rectangle is a square. When you name a shape, use the most exact name, so a square is called a square.

Is a circle a polygon?

No. A polygon has only straight sides. A circle has one curved edge and no corners, and so does an oval.

What is a shape with 7 sides called?

A heptagon. A shape with 9 sides is a nonagon, 10 sides a decagon and 12 sides a dodecagon. A 50p coin is nearly a heptagon, but its 7 edges are slightly curved.

Is there an easy way to remember pentagon?

Draw a house, a square with a triangle roof on top, and go round the outside. It has 5 sides, so the outline of the house is a pentagon.