The Yellow Emperor's Classic of Internal Medicine, specifically Chapter 51, details the proper depth of acupuncture needles. The text presents a dialogue between the Yellow Emperor and Qi Bo, exploring the precise placement of needles to target specific tissues (bone, tendon, muscle, vessel, skin) while avoiding damage to adjacent layers. Qi Bo emphasizes the importance of needle withdrawal before reaching the intended target layer to prevent injury, highlighting a principle of controlled and precise acupuncture technique. This emphasizes the need for careful and precise technique in acupuncture.