Here my dog "Flame" has actually her face made perfectly symmetrical through a bitof photo magic.

The white line under the center is theline of Symmetry


When the folded component sits perfect on peak (all edge matching), then the fold line is a heat of Symmetry.


Here I have actually folded a rectangle one way, and also it didn"t work.

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So this is not a line of Symmetry

But when I shot it this way, the 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 deserve to have 3, or 1 or no lines of symmetry:

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Equilateral Triangle(all sides equal, all angles equal) Isosceles Triangle(two political parties equal, 2 angles equal) Scalene Triangle(no political parties equal, no angle equal)
3 present of Symmetry1 line of SymmetryNo currently of Symmetry

Quadrilaterals

Different species of square (a 4-sided plane shape):

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Square(all political parties equal, all angle 90°) Rectangle(opposite sides equal, all angles 90°)Irregular Quadrilateral
4 lines of Symmetry2 present of SymmetryNo present of Symmetry

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KiteRhombus(all sides equal length)
1 heat of Symmetry2 present of Symmetry

Regular Polygons

A continuous polygon has all political parties equal, and all angle equal:

An Equilateral Triangle (3 sides) has 3 currently of Symmetry
A Square (4 sides) has 4 currently of Symmetry
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A Regular Pentagon (5 sides) has 5 present of Symmetry
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A Regular Hexagon (6 sides) has 6 currently of Symmetry
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A Regular Heptagon (7 sides) has 7 present of Symmetry
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A Regular Octagon (8 sides) has 8 currently of Symmetry

And the pattern continues:

A consistent polygon of 9 sides has actually 9 lines of SymmetryA regular polygon of 10 sides has actually 10 currently of Symmetry...A continuous polygon of "n" sides has "n" present of Symmetry

Circle