Dan's Conjoined Circles
Dan Anderson started my Monday off with this hypnotic image: http://recursiveprocessing.tumblr.com/post/134722191794/twin-circles-1x-2x-and-2x-1x-play-off-of-twin
Off to the GGB cave! This is a great structure to play with, and I really had to think about how to set the period so that it always completes the pattern.