"External Trinity" refers to the trunk of the four limbs of the body, that is, hand and foot are connected, elbow and knee are connected, and shoulder and hip are connected. Hands and feet should follow up and down when moving, and they should attract each other until they reach their hands. The combination of the opening and closing of yin and yang with both hands and the outward swinging and inward buckling of both feet is the premise of completing the elbow-wrapped crotch, and the elbow and knee are very important in Wushu. The combination of shoulder and hip is the key to keep the body upright, impartial and not leaning forward or backward. Shoulders and hips should be related, and hip straps should be consistent up and down. Its application is mainly in the waist, and the upper, middle and lower joints should be adapted. Three joints of limbs should be combined with crossed nerve veins, which is a combination skill of looking around and changing the compound strength. "Shoulder-hip joint", when the body changes, the shoulder well point of the upper limb root joint intersects with the jumping point of the lower limb root joint left and right; "Elbow and knee joint" is the intersection of Quchi point in the middle of upper limb and Yanglingquan point in the middle of lower limb; "Hand-foot joint" refers to the intersection of Laogong point at the tip of upper limb and Yongquan point at the tip of lower limb. Only in this way can we achieve the integrity of the body and the joint degree of the upper and lower limbs, and form three lines of defense: hand-foot joint, elbow-knee joint and shoulder-hip joint, which makes it difficult for opponents to attack. As the boxing theory says, "it is difficult to follow people up and down." The external trinity is not only the external foundation and condition for the operation of the internal spirit, but also the structural basis for the formation of the whole family, and it is also an important combination of internal forces. Therefore, in Tai Ji Chuan's training, Gong Zhuang's important purpose is to make the body gradually and naturally meet the requirements of the internal and external trinity, so that the internal and external trinity can be coordinated, thus achieving good results in both fitness and attack and defense. The combination of internal and external is both internal and external, and it cannot be neglected. Therefore, it is necessary to cultivate the spirit inside, the limbs outside, and both inside and outside, so as to achieve the unity of inside and outside.