In the previous book, a reflection was defined as a transformation where the pre-image is "flipped" across a line to obtain a mirror image. The line about which the figure is reflected is called the line of reflection.  This line is also called the line of symmetry because every point on the pre-image is the same distance from the line of reflection as its corresponding image point. The distance from a point to a line is measured with a line perpendicular to the given line.  Therefore, the reflection line is the perpendicular bisector of the segment connecting a pre-image point to its image point. 

For example, in the reflection below:



All reflections have opposite orientations from their pre-images.  In the drawing above reading the letters of the points in alphabetical order is done in a clockwise order.  Reading the image points in alphabetical order is done in a counter-clockwise order.

If the image is reflected across the same line of reflection, then its image will coincide with the original shape.  This is true for every reflection.