Clark University
Clark University: Dave's Short Trig Course: Transformations of the Plane
Site that focuses on planar pattern and transformations within geometry. It also identifies different kinds of symmetry.
CK-12 Foundation
Ck 12: Analysis: Two Dimensional Vectors
[Free Registration/Login may be required to access all resource tools.] Introduction to vector notation in two dimensions
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.