This Chapter from the Yellow Emperor's Classic of Internal Medicine details the cyclical flow of wei qi (defensive qi) through the body. Wei qi's daily circulation aligns with celestial movements, completing approximately fifty circuits, twenty-five through yang regions during the day and twenty-five through yin regions at night. The text describes the pathways of wei qi, its relationship to the twelve double-hours and twenty-eight lunar mansions, and the importance of timing acupuncture treatments according to its flow for optimal therapeutic effect. Seasonal variations and the specific location of wei qi at different times of day are also discussed. The text ultimately emphasizes the interconnectedness of celestial and human rhythms in maintaining health.