Tai Chi Chuan
The most well-known of the internal martial arts (Nei chia), this martial art is now mostly practiced as a form of exercise, popular because of its low-impact practices, but it is more importantly known as a fighting art.
History
The actual history of Tai Chi Chuan (commonly known as Tai Chi, and also spelled Taiji) has become a slightly contentious topic, as the legendary origin is often disputed.
According to the traditional story, the style began with Chang San-feng, a Shaolin monk. The story goes that somewhere in the 14th to 17th century, he travelled to Chen village, and inspired by seeing a crane defend itself against a snake, created a style that used roundness and fluidity to defend against even very strong strikes.
Historically speaking, Chen village is the agreed-upon genesis point of Tai Chi. As for Chang San-feng, his existence or role in Tai Chi is somewhat debatable, given that his very existence is a subject of disagreement. Additionally, people question why a Shaolin Monk would leave the temple in order to travel to a random village to teach a new art.
The counter theory is that Tai Chi evolved out of a Chen family boxing style, and the Chang San-feng association was incorporated as a way to tie the origin to an established point with a legenday figure, a common theme in the origin of martial arts; for instance the history of Hsing I has a similar debatable connection, which is used to establish lineage to a legendary figure.
A possible middle ground, which is considered quite plausible, is that Tai Chi evolved as a combination of the experience of a shaolin monk (which may or may not have been Chang San-feng) and an established family boxing style.
Characteristics and Training
Despite the new age translation of Tai Chi to a peaceful form of exercise, the form is very much a fighting art at heart. Both through practice of techniques and physical conditioning, Tai Chi practicioners have historicaly been pretty well-equipped for a fight.
To begin with, Tai Chi is an upright style with little to no groundwork. It is, however, a style with a large amount of stand-up grappling. Additionally, a wide variety of strikes are available.
As a philosohpy, Tai Chi is based on the concept of roundness. The most common analogy to the energies of Tai Chi is that of a ball; no matter what side the ball is struck from, it maintains its form and structure. In terms of defence, Tai Chi stresses even application of energy in all directions, punctuated by powerful strikes and throws once the fighter has moved themselves to within the opposing fighters defences.
Often, Tai Chi is characterized by statements like "if you speed it up, it's a fighting style". This is a half-truth; in fact, a practicioner attempts to match or control the speed of the opponent through contact and grappling, drawing the opponent in until the opponent is controlled by the practicioner and then thrown or struck, often with great force. One does move faster as much as one forces the opponent to move slower; as such, Tai Chi is an art that requires making and maintaining contact in order to fight properly. This need to move within and stay within short range, plus the subtlety of many of the techniques, makes Tai Chi an art with a rather lengthy learning curve.
Tai Chi is practiced using sets or forms; most have either one or two forms. Depending on the style these forms have a certain number of movements within them, often 108, but shorter forms have evolved as well. These forms represent the set of movements used by the practicioner, and these moves are applied and adapted to form the fighting art.
In addition to daily practice of the forms (and practicing/repeating the same movements), other training exercises are also done. Among them are push hands, a paired exercise which is meant to teach both praciticioners sensitivety to others movement and also acts as a form of mild competition. Besides push hands, Chi Kung exercises are done to increase strength, and other exercise regimens adapted to the specific movements of Tai Chi are often incorporated.
Styles
Tai Chi Chuan practicioners belong to one or more styles of the art. These styles are also characterized by lineage; that is, the teaching line between themselves and the masters of the art.
The major fighting styles are Chen, Yang, Wu, and Sun, all based on the family names of the respective originator or master. Of these, Chen style is the originator form (though of course it's changed significantly over time) and is considered the most overtly martial in form. Yang style is the most common fighting style of Tai Chi Chuan out there. There are also a myriad of smaller styles, often a single lineage.