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What is a line of symmetry?

Part of Mathematics and NumeracyPractise and revise KS2 maths

Introduction to symmetry and tessellations

A butterfly with a line of symmetry down the centre image used for preparation for the SEAG transfer test

This page has been put together to help you practise and revisit some of the brilliant skills you’ve learned all through primary school.

It’s a great way to boost your confidence in Maths and get you ready for the exciting next step into Year 8!

Let's look at shapes, patterns, and reflections!

In this guide, you will learn:

  • what a line of symmetry is (a 'fold line' or 'mirror line' that makes two halves match perfectly)
  • which common shapes have lines of symmetry and why a square has more than a rectangle
  • how symmetry is used in architecture, nature, and art
  • what tessellations are and how they link to transformations (flips, slides, and turns)
A butterfly with a line of symmetry down the centre image used for preparation for the SEAG transfer test
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What is a line of symmetry?

A shape has a line of symmetry if you can draw a straight line through it (like a fold) and one side becomes a perfect mirror image of the other.

A rectangle has 2 lines of symmetry (one vertical, one horizontal).

A square has 4 lines of symmetry (one vertical, one horizontal, and two diagonal).

Why does a square have more? A square has diagonal lines of symmetry because all its sides are the same length. When you fold it diagonally, the corners match up perfectly. A rectangle's sides are not all the same length, so folding it diagonally doesn't work!

You can see symmetry all around you, from the wings of a butterfly to the windows on a building.

A diagram of a yellow rectangle and a turquoise square showing the lines of symmetry. The rectangle has two lines of symmetry and the square has four.
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Tessellations and transformations

Six different tessellated patterns

A tessellation is a pattern made of one or more shapes that fit together perfectly with no gaps or overlaps, like tiles on a floor or the pattern on a brick wall.

These patterns are made using three "transformations" (ways to move a shape):

  • Translation (slide): The shape simply slides to a new position without flipping or turning. (eg a brick wall pattern).

  • Reflection (flip): The shape is flipped over a mirror line (a line of symmetry) to create the next part of the pattern.

  • Rotation (turn): The shape is turned around a fixed point.

Image demonstrating translation. A yellow L shape is shown to the left. Four squares to the right and one square down is a turquoise L shape.
Image caption,
Translation
Image demonstrating reflection. A lilac L shape on the left is reflected in a mirror line - a yellow backwards L shape shows the reflection on the right of the mirror line.
Image caption,
Reflection
Image demonstrating rotation. A yellow L shape is shown on the left. Rotated by 90 degrees and moved four squares to the right is another yellow L but on its side this time.
Image caption,
Rotation
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Symmetry Activity

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Transformation detective

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Summary

A boy learning about symmetry for the SEAG transfer test

Fantastic! You are now a symmetry expert.

You know that a line of symmetry is a mirror fold and you can explain why a square has more lines than a rectangle.

You can also spot the transformations (flips, slides, and turns) used to make tessellating patterns.

A boy learning about symmetry for the SEAG transfer test
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Brain boost

Think about these questions to stretch your thinking and sharpen your skills!

  • Think of three capital letters of the alphabet. How many lines of symmetry does each one have? (eg A, H, T, B, F)

  • Why does a perfect circle have 'infinite' (too many to count) lines of symmetry?

  • Look at the pattern on a brick wall. What shape is tessellating? What transformation (slide, flip, or turn) is used to make the pattern?

Have a chat about your answers with a parent, teacher or your class.

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