Here my dog "Flame" has her face made perfectly symmetrical with a bitof photo magic.
The white line down the center is theLine of Symmetry
When the folded part sits perfectly on top (all edges matching), then the fold line is a Line of Symmetry.
Here I have folded a rectangle one way, and it didn"t work.
But when I try it this way, it does work (the folded part sits perfectly on top, all edges matching):
Triangles
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 |
Quadrilaterals
Different types of Quadrilaterals (a 4-sided plane shape):
 |  | |||
| Square(all sides equal, all angles 90°) | Rectangle(opposite sides equal, all angles 90°) | Irregular Quadrilateral | ||
| 4 Lines of Symmetry | 2 Lines of Symmetry | No Lines of Symmetry |
 |  | |
| Kite | Rhombus(all sides equal length) | |
| 1 Line of Symmetry | 2 Lines of Symmetry |
Regular Polygons
A regular polygon has all sides equal, and all angles equal:
| An Equilateral Triangle (3 sides) has 3 Lines of Symmetry | ||
| A Square (4 sides) has 4 Lines of Symmetry | ||
 | A Regular Pentagon (5 sides) has 5 Lines of Symmetry | |
 | A Regular Hexagon (6 sides) has 6 Lines of Symmetry | |
 | A Regular Heptagon (7 sides) has 7 Lines of Symmetry | |
 | A Regular Octagon (8 sides) has 8 Lines of Symmetry |
And the pattern continues:
A regular polygon of 9 sides has 9 Lines of SymmetryA regular polygon of 10 sides has 10 Lines of Symmetry...A regular polygon of "n" sides has "n" Lines of SymmetryCircle |