The twelve channels follow fixed directions. The three yin channels of the hand run from the chest to the hand, the three yang channels of the hand from the hand to the head, the three yang channels of the foot from the head to the foot, and the three yin channels of the foot from the foot to the chest. Each channel connects to the next at a specific point, producing a continuous circuit through which qi and blood circulate.
These rules are not academic. They determine where a disorder is likely to travel, and they underlie the practice of selecting a point on one channel to affect another. A headache attributed to the stomach channel, for example, is treated at a point on the leg because that is how the channel runs.