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In every person, the two hands and two feet each possess three Yin meridians and three Yang meridians, which combine to form the Twelve Primary Meridians. The Three Yin meridians are Taiyin (Greater Yin), Shaoyin (Lesser Yin), and Jueyin (Terminal Yin); the Three Yang meridians are Taiyang (Greater Yang), Yangming (Yang Brightness), and Shaoyang (Lesser Yang). The three Yin and three Yang meridians in each hand and foot combine to constitute the Twelve Primary Meridians. They are called "Meridians" (Jing) because Qi and Blood flow through them continuously without cessation; they are called "Vessels" (Mai) because they represent the channels through which blood distributes and circulates throughout the body.

The Three Yin meridians of the hands travel from the Zang organs to the hands; the Three Yang meridians of the hands travel from the hands to the head. The Three Yang meridians of the feet travel from the head down to the feet; the Three Yin meridians of the feet travel from the feet up to the abdomen.

The Three Yin meridians of the hands originate from the Zang organs and proceed to the hands. Specifically: Hand-Taiyin Lung Meridian originates from the Middle Jiao (the region of the Spleen and Stomach) and terminates at the tip of the thumb; Hand-Shaoyin Heart Meridian originates from the Heart and terminates at the tip of the little finger; Hand-Jueyin Pericardium Meridian originates from the Chest and terminates at the tip of the middle finger.

The Three Yang meridians of the hands travel from the hands to the head. Specifically: Hand-Yangming Large Intestine Meridian originates at the radial side of the index finger (adjacent to the thumb) and ascends to flank the nostrils; Hand-Taiyang Small Intestine Meridian originates at the tip of the little finger and ascends to the inner canthus of the eye; Hand-Shaoyang Sanjiao Meridian originates at the ulnar side of the ring finger (adjacent to the little finger) and ascends to the outer canthus of the eye.

The Three Yin meridians of the feet enter the abdomen from the feet. Specifically: Foot-Taiyin Spleen Meridian originates at the tip of the big toe, ascends to connect with the Spleen and link to the Stomach; Foot-Shaoyin Kidney Meridian originates at the sole of the foot, ascends to connect with the Kidney and link to the Bladder; Foot-Jueyin Liver Meridian originates at the hair cluster on the big toe, ascends to connect with the Liver and link to the Gallbladder. Although the Three Yin meridians of the feet are said to enter the abdomen from the feet, Foot-Taiyin Spleen Meridian further ascends through the diaphragm, flanks the pharynx, and disperses below the root of the tongue; Foot-Shaoyin Kidney Meridian further ascends from the Kidney to flank the root of the tongue; Foot-Jueyin Liver Meridian further ascends to emerge from the forehead and meets the Du Vessel (Governing Vessel) at the vertex of the head.

Generally speaking, Hand-Taiyin Lung Meridian emerges horizontally from the Lung system (trachea and throat tissues) into the axilla; Hand-Shaoyin Heart Meridian ascends from the Heart system through the Lungs and emerges into the axilla; Hand-Jueyin Pericardium Meridian follows the hypochondrium upward to reach the axilla. This is what Qin Yue-ren (Bian Que) referred to when stating that all Yin meridians reach the neck and chest before returning, whereas the Jueyin meridian further ascends to emerge at the vertex. This is because Jueyin represents the extremity of Yin. The reason for this is to demonstrate the principle that Yin has no true end, mirroring the concept in the *I Ching* (Book of Changes) of "unconsumed fruit," which illustrates that Yang also has no end. However, the Yin and Yang in the *I Ching* are discussed in terms of Qi, while Yin and Yang in the human body are discussed in terms of Zang-Xiang (visceral manifestations). Qi is formless, whereas Zang-Xiang have physical substance; Qi is Yang, and substance is Yin. Therefore, does the formless favor Yang, and the substantial favor Yin?