Root and knot of meridians and collaterals are as installations. Root installs at tip of each meridian course and its position is at upper limbs and lower limbs. Knot installs at position which assembles qi and blood in each meridian course and its position is that yang meridians are at head and yin meridians are at chest and abdomen. Root and knot are as spigot of switch and control running of qi in meridians and collaterals. Spigot is divided into yin and yang. Tai yang meridians are in charge of open and yin yangming meridians are charge of close, shaoyang meridians are in charge of hinge between open and close in spigot of yang meridians. Tai yin meridians are in charge of open and jueyin meridians are charge of close, shaoyin meridians are in charge of hinge between open and close in spigot of yin meridians.

Function of yin and yang spigots:
Regulate QI-blood circulation quantity of three congeneric meridians.

Regulate degree of yin and yang attribute and keep attribute of each meridian temperate process of taiyang, shaoyang, yangming, taiyin, shaoyin and jueyin.

Guarantee normal dunning of qi and blood and do not make qi and blood flow backwards, cross flow and run off.

Prevent invasion of exogenous evil, if meet exogenous evil it will close and keep exogenous evil out of meridians. If spigot is malfunction, when it should close but not shut and when it should be open but is imperforation, it will contract disease. It illustrates that if spigot loses function, qi of yin and yang will be disorder and lose seriously then lose defense function and contract immedicable disease.