"A Yinmi Sign... I have seen these before, but never understood them." -Shinichi Hazuki.
Yinmi Signs are part of a code martial artists use to secretly communicate with each other. It uses four round objects, most often cups, left in a public place or place a martial artist may find themselves in need of a message in this code. The arrangement, left to right, are represented in writing as the Wukongese characters for Heaven, Earth, Dragon, and Comrade respectively. Heaven stands for time, Earth for place, Dragon for intent, and Comrade for the intended recipient of the message.