Nomenclature of the twelve main meridians
The twelve main meridians are of four groups: three yin meridians of hand, three yang meridians of hand, three yin meridians of foot and three yang meridians of foot. Three yin meridians of hand include Taiyin Lung Meridian of Hand, Shaoyin Heart Meridian of Hand and Jueyin Pericardium Meridian of Hand. Three yang meridians of hand include Shaoyang Sanjiao Meridian of Hand, Taiyang Small Intestine Meridian of Hand and Yangming Large Intestine Meridian of Hand. Three yang meridians of foot include Shaoyang Gall Bladder Meridian of Foot, Taiyang Bladder Meridian of Foot and Yangming Stomach Meridian of Foot. Three yin meridians of the foot include Taiyin Spleen Meridian of Foot, Shaoyin Kidney Meridian of Foot and Jueyin Liver Meridian of Foot.
The twelve main meridians are distributed along the body symmetrically and run along the medial or lateral sides of the four limbs, with each meridian pertaining to one zang organ or fu organ. Therefore each of the meridians is named according to the nature of yin or yang, the medial or the lateral side of hand or foot, and the names of zang or fu to which they pertain.
A. Hand or foot
The meridians running along the upper limbs are named "hand”, while those running along the lower limbs are named “foot”.
B. Yin or yang
The meridians going in the medial aspect of the four limbs are named “yin”, whereas those in the lateral aspects are named "yang”. According to the three divisions of yin-yang, one yin can be subdivided into three yins: namely taiyin, jueyin, and shaoyin, which are respectively distributed along the anterior border, midline and the posterior border of medial sides of the limbs (in the medial sides of the lower limb, at 8cun above the medial malleoli, jueyin is located in the anterior, taiyin in the middle). Accordingly one yang can be subdivided into three yangs: yangming, shaoyang and taiyang,whieh are respectively distributed along the anterior border, midline and the posterior border of lateral sides of the lower limbs.
C. Zang or fu
Each yin meridian pertains to one zang organ while each yang meridian to one fu organ. The nomenclature of zang or fu is determined in the light of the name of zang or fu to which a meridian pertains.