In this example, the quadrilateral ACDB has been reflected over the vertical segment, which serves at the reflection line, to form the image A'C'D'B'. Now, we can connect a point, A for example, and its image,
A'. Then point P is the point of intersection of the reflection line and segment AA', and it is the midpoint of the segment
AA'. Also, the reflection line is perpendicular to the segment AA'. Thus, the reflection line is the perpendicular bisector of segment AA'.