The 19th century artist/writer/interior designer created many beautiful wallpaper patterns.
You can find out more about him at the William Morris homepage posted by the English government.
Click on a thumbnail to see a larger version of the pattern.
Look for lines of symmetry and translated elements.
The Dutch Artist Maurice Escher also created many fascinating wallpaper patterns.
See some of his work at these web pages.
http://www.worldofescher.com/store/tessftp.html
http://ftp.sunet.se/pub/pictures/art/M.C.Escher/
http://www.iproject.com/escher/escher100.html
Official M.C. Escher Site
http://www.mcescher.com/