Some started life as regular doodles that got scanned in and touched up. Others started out in silico.
At MacWorld Expo '97 in Boston, the keynote addresses were held in an old armory, called "The Castle". On the outside, it looked like a small castle. On the inside, it looked like a big, dark room. Around the top of the auditorium, there was a narrow walkway with a wrought iron fence. I copied down the intricate pattern of the fence.
A pattern suitable for background tiling.