When people speak of Tai Chi, usually they speak of it as a single style, even (in my experience) among those who are knowledgeable about martial arts. At first look, they do look very similar in that they each catalog and teach its techniques through the use of extensive forms practice at slow paces. They seem to have very similar techniques. So what makes one style of Tai Chi distinct from another?
- Chen-style has explosive movements (fajin), jumps, directional changes.. It is the style of Daiji closed to Bagua.
- The Yang style is more flowing, has bigger and more open postures (longer stances, arms open wider). It has a slow form and a fast form, thus separating concepts that are found together in the Chen forms.
- Wu style has a higher stances and smaller circles than Yang. It is also sometimes called Hao style.
- Sun style is in fact a combination of Xing-Yi, Bagua and Wu-style Daiji created by Sun Lu Tang, so it has some properties of each: the more evasive movements of Bagua and the more "martial flavor" of Xing-Yi.
- Wujian style is very close to the Yang style but has higher stances and a recognizable leaning forward of the torso in most of the movements.
The above is a summary of information found in a compact yet readable manner on this website, which I really recommend as a read on TaiChi, as it has a lot of very good information on Bagua, Xing-Yi, and some fairly fundamental concepts including how to stand / good posture and a page on the philosophy of martial arts.