A line of symmetry is a line through a shape. It shows that on either side of this line that the picture is exactly the same. There can be more than one line of symmetry in a shape.
They are 'mirror images' when a shapes are divided by a lines of symmetry.
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For a 2-dimensional object, a line of symmetry is a line such that, if you were to fold the object along that line, the two sides would match up exactly.
For a 3-dimensional object, a line (axis) of symmetry is a line such that, if you were to rotate the object around it, its shape would remain the same.
When if you split something in half, it will look like one of the sides is looking into a mirror.
2 lines of symmetry
A nephroid has 2 lines of symmetry.
A parallelagram can be a square, which has four lines of symmetry or a rectangle which has two lines of symmetry but the generic parallelagram has zero lines of symmetry
5 lines
A decagon need not have any lines of symmetry. It can also have 1 or 10 lines of symmetry.