From the site's introduction: "From 1938 to 1943, the Dutch artist M.C.Escher experimented with periodic tiling of the plane using a simple motif carved into a wooden block. He rotated the block 90 degrees three times and labelled the four images thus created as 1, 2, 3 and 4. Using a mirror image block and also two other blocks with the pattern going 'over' instead of 'under', he had 16 tiles at his disposal. He would then select 4 of these blocks, arranged 2 by 2, and use this 'big tile' as a stamp to create patterns. He hand coloured some of these patterns." The applet doesn't allow you to color the patterns, but you can save the image to your hard drive and then use any paint application (even MS paint) to fill in the colors.