The fifteen collaterals is a collective term for the collaterals of the twelve meridians, the governor and conception vessels as well as the major collateral of the spleen.
The cyclical running of the twelve collaterals is composed of two parts: after stemming from the “Luo-Connecting” acupoints located below the knees and elbows, one part runs to the meridian that it is in internal and external relation with; the other part runs parallel to the meridian that it stems from and associates with certain viscera, organ and region. Collaterals are fine and numerous, making it difficult to locate and nominate. But the collaterals of the twelve meridians have definite location. They are connected with the meridians in external and internal relation on one hand, and run parallel to the meridians sharing both the properties of the meridians and collaterals on the other.
That is why they are called “major collaterals”. The three collaterals over the trunk of the body distribute on the front, back and side of the body. The major collaterals of the twelve meridians further strengthen the association of the meridians in external and internal relation below the knees and elbows. It is also marked by definite distribution, association with certain viscera and organs and improvement of the distribution of the twelve meridians. The three collaterals on the trunk of the body mainly function to infuse qi and blood in the front, back and side of the body.
 Both the meridian branches and collaterals function to strengthen the association of the meridians in external and internal relation. The difference is that the meridian branches mainly strengthen the association of the viscera in external and internal relation as well as the association of the meridians in external and internal relation on the head and face; while the collaterals mainly reinforce the association of the meridians in external and internal relation below the knees and elbows.