Answer:

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

symmetry folding yes

So this is a Line of Symmetry

Triangles

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

symmetry equilateral triangle symmetry isosceles triangle no symmetry scalene triangle

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):

symmetry square symmetry RECTANGLE symmetry IRREGULAR quad

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

symmetry kite symmetry rhombus

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:

symmetry equilateral triangle An Equilateral Triangle (3 sides)

has 3 Lines of Symmetry

symmetry square A Square (4 sides)

has 4 Lines of Symmetry

symmetry regular PENTAGON A Regular Pentagon (5 sides)

has 5 Lines of Symmetry

symmetry regular hexagon A Regular Hexagon (6 sides)

has 6 Lines of Symmetry

symmetry regular septagon A Regular Heptagon (7 sides)

has 7 Lines of Symmetry

symmetry regular octagon A Regular Octagon (8 sides)

has 8 Lines of Symmetry

And the PATTERN continues:

A regular polygon of 9 sides has 9 Lines of Symmetry

A regular polygon of 10 sides has 10 Lines of Symmetry

...

A regular polygon of "n" sides has "n" Lines of Symmetry