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.

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So this is not a Line of Symmetry

But when I try it this way, it does work (the folded part sits perfectly on top, all edges matching):

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So this is a Line of Symmetry

Triangles

A Triangle can have 3, or 1 or no lines of symmetry:

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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 Symmetry1 Line of SymmetryNo Lines of Symmetry

Quadrilaterals

Different types of Quadrilaterals (a 4-sided plane shape):

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Square(all sides equal, all angles 90°) Rectangle(opposite sides equal, all angles 90°)Irregular Quadrilateral
4 Lines of Symmetry2 Lines of SymmetryNo Lines of Symmetry

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KiteRhombus(all sides equal length)
1 Line of Symmetry2 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
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A Regular Pentagon (5 sides) has 5 Lines of Symmetry
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A Regular Hexagon (6 sides) has 6 Lines of Symmetry
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A Regular Heptagon (7 sides) has 7 Lines of Symmetry
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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 Symmetry

Circle