The terms biao li may be translated as outer and inner, exterior and interior, and they must be differentiated from the similar terms nei (內) and wai (外). In medicine biao/li is often used to indicate a movement of qi, which is either yin (towards the interior) or yang (towards the exterior).

The etymology of the characters can be seen in the radical they both share – that for cloth or clothing (衣). The biao (表) of a garment is its external outer appearance, the li (裏) its lining, or hidden aspect. The biao faces towards the exterior and can be seen, the li faces towards the inner and cannot be seen. Biao can also mean to be manifest, and the interesting interplay of these two characters expresses the way in which inner qualities and conditions can manifest themselves at the exterior – a very important aspect of diagnosis.

Within classical medicine, biao (表) often refers to the movement from the depths of the body towards the exterior, and by extension the ability to secure that movement of qi in the action of defence (wei 微). Li (裏) is the movement towards the interior, the ability to draw and keep qi within, in order to nourish and rebuild (ying 營). As with all yin and yang couples, these actions must balance and regulate each other. The movement towards the exterior must always be anchored in the yin, or the qi may dissipate. Similarly the movement towards the interior must be balanced by the yang, which provides movement and circulation – preventing stasis and stagnation.

This action and interaction is beautifully illustrated by the function and inter-relationship of the extraordinary meridians the yin and yang wei mai (維 脈).

Classical dictionaries suggest that the character li (裏) may be substituted by another (裡), and this is commonly found in the medical literature. The use of this character gives us an interesting connection to another li (理) which shares the same phonetic (里) but has the radical for jade (玉) instead of that for emanation or manifestation (礻). This well known but illusive character describes an inner structure, a texture or pattern, and later came to represent innate laws and principles.

In classical medical texts, the kidneys are often said to guard the li and to gather together all the elements necessary to maintain the inner structure of the body. Neijing suwen chapter 52 says that kidneys govern the li (裡), the heart the biao (表), reflecting the action of the water qi, which moves downwards towards the depths of the body, and the fire, which moves upwards with a yang movement of dispersion. This shaoyin couple govern both the inner structure and the more outer circulation, the consolidation of essences and the radiation of the spirit.