Axes of Symmetry
The axis of symmetry of a shape is a straight line that divides it into two identical parts: fold the shape along that line and both halves coincide exactly. A shape may have one axis, many axes, or none at all.
Regular polygons: axes = sides
Note: the number of axes of symmetry of a regular polygon equals its number of sides.
Shapes to remember
Careful: irregular shapes
Having equal sides is not enough. A rhombus has four equal sides but only 2 axes of symmetry, because its angles are not equal. A rectangle has equal angles but only 2 axes, because its sides are not all equal.
A parallelogram that is neither a rectangle nor a rhombus has no axis of symmetry at all — although it does have rotational symmetry of order 2.
Test to use: fold along the candidate line. If every vertex lands exactly on another vertex, the line is an axis of symmetry.