Identifying 2D shapes - KS2 Maths - Year 5

2D shapes

Two-dimensional (2D) shapes are completely flat. They have two dimensions – length and width.

A polygon is a closed 2D shape with straight sides that meet at vertices. Polygons can be regular or irregular.

If all the straight sides of a shape are the same length and interior angles are the same size, then it is a regular polygon.

If the length of the sides are different and the angles are not the same size, then it is an irregular polygon.

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A property is a quality that a shape has.

Examples of shape properties are:

You can use a shape's properties to help identify and classify it.

Polygons are named depending on the number of sides.

A polygon is a 2D shape with straight sides that meet at vertices.

Polygons have three or more sides. A circle is not a polygon because it doesn’t have straight sides.

Polygons can be regular or irregular.

A regular polygon has sides that are equal in length and equal angles.

These shapes are all regular polygons.

A regular triangle is called an equilateral triangle. A regular quadrilateral is called a square.

Irregular polygons are shapes that have sides and angles of different sizes.

You can always identify the polygon by how many sides it has. For example a pentagon is always a shape that has 5 sides.

A triangle is a 2D shape with three sides.

There are four different triangles:

Image caption,
Equilateral triangle
An equilateral triangle has 3 sides of equal length. The dashes on the lines show they are equal in length. All of the angles are also equal.
Image caption,
Isosceles triangle
An isosceles triangle has 2 sides of equal length. The angles at the base of the equal sides are equal.
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Scalene triangle
A scalene triangle has 3 sides of different lengths and 3 unequal angles.
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Right-angled triangle
A right-angled triangle is a triangle that has a right angle.

The angles inside a shape are called interior angles.

The interior angles in a triangle always add up to 180°.

You can use this facts to calculate missing angles in a triangle.

Take a look at this triangle. The angle labelled b is missing.

You know that one of the angles is 50°. The angle marked with a little square indicates that it is 90°.

You know that the interior angles of a triangle should add up to 180°. Therefore, you can subtract these values from 180° to find the missing angle.

180° - 90° - 50° = 40°

So angle b is 40°.

A quadrilateral is a 2D shape with four sides.

There are six special quadrilaterals with different properties.

Image caption,
Rectangle
A rectangle has two pairs of equal parallel sides. The interior angles are all right angles.
Image caption,
Sqaure
A square is a special type of rectangle. It has four equal parallel sides and four angles that are right angles.
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Parallelogram
A parallelogram has 2 pairs of equal sides. The interior angles that are diagonally opposite each other are equal.
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Trapezium
A trapezium has one pair of parallel sides.
Image caption,
Rhombus
A rhombus has four sides of equal length. It has two pairs of equal angles. The angles that are diagonally opposite each other are equal.
Image caption,
Kite
A kite has two pairs of sides of equal length. It has one pair of equal angles.