Forum for Traditional on this very topic. The whole post may be read here.
Usually, the beginner first learns the slow form of Taijiquan. Anyone who at this stage is interested in the classic texts of Taijiquan will find time and time again the notion of the 13 basic movements. So what are these 13 basic movements? In Chinese they are called shisanshi.
Shisan is the number 13 and the second shi means basic movement. In a direct translation shi is given as “posture”, “position”, “gesture” or, as in Sunzi, “strategic advantage”. When talking about Taijiquan it is best to use the word “movement” to express the dynamic character of shi. The 13 basic movements are subdivided into eight hand techniques (bamen,literally: eight gates) and five steps (wubu). The eight hand techniques are allocated to the compass points respectively to the eight trigrams. The five steps are allocated to the five phases (wuxing). The eight directions are in China traditionally the four sides North, South, East and West and the four corners, NE, SE, SW, NW. Together, these make up the 13 basic movements of Taijiquan.
They are explained in the “Explanation of the method of Taijiquan (Taiji fashuo)” in text 1:
The eight hand techniques and the five steps (Bamen wubu)
Direction Eight Gates
peng South kan
lü West li
ji East dui
an North zhen
cai Northwest xun
lie Southeast qian
zhou Northeast kun
kao Southwest gen
“The compass points and the eight hand techniques demonstrate the law of the cyclic change between yin and yang, which changes eternally. In brief, one has to learn the four sides and the four corners. Peng, lü, ji and an are the hand techniques of the four sides. Cai, lie, zhou and kao are the four hand techniques of the four corners. Combining the hand techniques of the four sides and the four corners we achieve the allocation of the gates to the trigrams.
The differentiation between five steps (wubu) is based on the idea of the five phases (wuxing) and supports the eight directions. The five phases are: jinbu (to advance)/fire, tuibu (to retreat)/water; (to look left)/wood, youpan (to look right)/metal; zhongding (central equilibrium), the centre of the directions/earth. Advancing and retreating are the steps of water and fire and to look left and right are the steps of metal and wood. The central equilibrium of the earth is the central point of the axis. The eight trigrams are hidden in the body, and the feet step the five phases. The eight hand techniques and five steps make 13. This is how the 13 basic movements are created naturally and are called the eight gates and the five steps.“ (Wu, p. 16).
The 13 movements are the basis of Taijiquan.