Let's see how we can put it to work.
Given a figure and its reflection, construct the line of reflection. This example chose to connect E to E'.
The Reflection of X-Rays by Crystals
You could, however, have chosen to connect D to D' or F to F' for your construction. It is sufficient to choose only one set of corresponding points to get the line of reflection. You need not bisect all three connected segments, unless you want to test the accuracy of your construction. Given a figure and line of reflection, construct the reflected image. Further reading Comparison of quartz crystallographic preferred orientations identified with optical fabric analysis, electron backscatter and neutron diffraction techniques N.