Vertically 1–1.5 cun.
Xuanzhong GB-39 has various functions. As the Gathering-Hui point for Marrow, its most important one is to nourish the Marrow. In fact, it is more effective in filling the bone marrow rather than the marrow in the Brain. Thus it is able to treat chronic Painful Obstruction Syndrome (Bi Syndrome) on bones and sinews due to Liver and Kidney Deficiency. Many practitioners use Xuanzhong GB-39 in old people to prevent Wind-stroke.
Furthermore, it is widely used to remove obstructions from Lessor Yang Channels (Gall Bladder and Triple Burner Channel) due to Wind invasion. Both Channels run through the lateral side of the neck and Obstructions there cause neck pain and stiffness. Sometime the patients can not even turn their head from side to side.
Another function is to subdue Liver Yang Rising, characterized by headache, dizziness and stiff neck.