Objects can be reflected on a mirror line or a axis of reflection to produce an image of the object. In these scenarios, each point in the image must be the same distance from the mirror line as the corresponding point of the original object.
When a shape such as a quadrilateral ABCD as shown below is reflected along a mirror line, the opposing side is labelled the image quadrilateral A’B’C’D’.
The image above, is said to be congruent to the original shape. Note also that when lines are drawn between certain points in the image, these lines are called perpendicular bisector lines.