Wikimedia
Wikipedia: Plane of Symmetry
This site from the free encyclopedia, Wikipedia, provides a basic definition for plane of symmetry. Links are given for additional information on related subjects.
Illustrative Mathematics
Illustrative Mathematics: G Co Seven Circles Ii
In this task, seven pennies are placed in a circle so that they are all touching one in the center and their neighbors. Learners are asked to find as many rigid motions of the plane as they can which are symmetries of this configuration...