Lines of symmetry in 2D shapes

In a nutshell

A 2D shape is symmetrical if you can fold it in half and the two halves are exactly alike. The line separating the identical halves is called the line of symmetry. It is also called the mirror line since either side of the line is the mirror image of the other. Some shapes have more than one line of symmetry.

Lines of symmetry

A line of symmetry is an imaginary line you can draw across a shape, where either side of the line will look identical. This is also called the mirror line because each side is a reflection of the other, like if you held a mirror to it.

Example 1

Is this shape symmetrical?

Maths; Geometry - properties of shapes; KS2 Year 4; Lines of symmetry in 2D shapes

You can draw a line across the shape and either side will be a mirror image of the other.

Yes, this shape is symmetrical.

Example 2

Is this shape symmetrical?

Maths; Geometry - properties of shapes; KS2 Year 4; Lines of symmetry in 2D shapes

You cannot draw a line across the shape which would leave either side as a mirror image of the other.

$\underline{No,\ this\ shape\ is\ not\ symmetrical.}$

Regular shapes

Regular shapes have all equal sides and equal angles. The lines of symmetry for a regular shape is equal to the number of its sides.

Shape	Number of sides	lines of symmetry
Equilateral triangle	$3$
Square	$4$
Pentagon	$5$
Hexagon	$6$

Irregular shapes

Irregular shapes have different sized sides and angles. Some also have lines of symmetry.

Shape	lines of symmetry	diagram
Rectangle	$2$
Rhombus	$2$
Isosceles triangle	$1$