What is a line of symmetry?
A line of symmetry divides a figure into two halves that are the mirror images of each other.
Fold a square sheet of paper exactly in half. When you unfold the paper you will see the crease down the center. That is an example of a line of symmetry. Both sides of an object must be equal to be symmetrical. Let's do a construction taking off on this idea. You've probably already done it at one time or another.
Fold that piece of paper, and take a pair of scissors and cut half a heart out of it using the crease as a line going down the "middle" of the heart. Unfold the finished construction. You'll have a heart and that fold you made in the paper is the line of symmetry for the figure. The line of symmetry divides any shape into mirror images.
A line which divides a body into two parts, such that each part is the mirror image of the others, then the line is called a line of symmetry. If such a line is parallel to the horizontal plane, then it is called a horizontal line of symmetry. Else, if the line of symmetry is perpendicular to the horizontal plane then it is a vertical line of symmetry.