Wallpaper Groups
Symmetry group 10  (p4)
This is the first group with a 90° rotation, that is, a rotation of order 4.  It also has rotations of order 2, that is, half-turns.  The centers of the half-turns are midway between the centers of the order-4 rotations.  There are neither reflections nor glide-reflections.  The lattice is square, and a quarter of a square fundamental region for the translation group is a fundamental region for the symmetry group.
 

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© 1994, 1997.
David E. Joyce 
Department of Mathematics and Computer Science
Clark University
Worcester, MA 01610
 
These files are located at http://aleph0.clarku.edu/~djoyce/wallpaper/