The symmetry axis of an angle is the straight line where the bisector of the angle lies.
The symmetry axis of an isosceles triangle is the line where the bisector of the top angle or the midline of the bottom edge or the height of the bottom edge lies.
The symmetry axis of an equilateral triangle is the line where the bisector of any angle is located or the line where the middle line of any side and the height of the base are located.
The symmetry axis of a square is the straight line passing through the midpoint of the opposite side and the diagonal line.
The symmetry axis of a rectangle is a straight line passing through the midpoint of the opposite side.
The symmetry axis of an isosceles trapezoid is a straight line passing through the midpoint of two bases.
The symmetry axis of a circle is any straight line passing through the center of the circle or a straight line with radius and diameter.