Reflection Symmetry

Reflection Symmetry (sometimes called Line Symmetry or Mirror Symmetry) is easy to recognise, because one half is the reflection of the other half.

Here my dog "Flame" has her face made perfectly
symmetrical with a bit of photo magic.

The white line down the center is the Line of Symmetry
(also called the "Mirror Line")

The reflection in this lake also has symmetry, but in this case:

the Line of Symmetry runs left-to-right
it is not perfect symmetry, because the image is changed a little by the lake surface.

Line of Symmetry

The Line of Symmetry (also called the Mirror Line) does not have to be up-down or left-right, it can be in any direction.

But there are four common directions, and they are named for the line they make on the standard XY graph.

See these examples (the artwork was made using Symmetry Artist) :

Line of Symmetry	Sample Artwork	Example Shape

Plane Shapes

Not all shapes have lines of symmetry, or they may have several lines of symmetry. For example, a Triangle can have 3, or 1 or no lines of symmetry:


Equilateral Triangle (all sides equal, all angles equal)	Isosceles Triangle (two sides equal, two angles equal)	Scalene Triangle (no sides equal, no angles equal)
3 Lines of Symmetry	1 Line of Symmetry	No Lines of Symmetry

I have collected some more examples at Lines of Symmetry of Plane Shapes.