This is the BASE SYSTEM for Bruce Lee's personal style.

The Center-Line Theory is an integral part of Wing Chun. It is the nucleus on which the defenses and attacks are based.

The Immovable Elbow Theory is very important in Wing Chun. In theory, it works like a hurricane. The eye of a hurricane is always still, yet it's periphery force is constantly moving and exerting tremendous force. The Immovable Elbow works the same way. The hand and forearm can move in any direction, but the elbow never moves. The lead elbow remains about 3 inches in front of the body on the CENTERLINE. The hand and forearm should never pass the imaginary perpendicular line that intersects the elbow. If the arm is pressed too hard, it is better to give way with the whole body than to give way with the elbow or violate the boundary line. The lead elbow is in such a position as to allow the lead hand to be on the centerline at approximately the location of the middle sternum. The back elbow is in such a position as to allow the back hand to be on the centerline location of the lower end of the sternum, parallel to the lead forearm.
In the immovable elbow theory, boundary lines limit the sideward and up and down movement of the hand and forearm. The high point of the boundary is the eye-brows, and the low point is the groin area, although the elbows must never dip below the navel. The width of the boundary extends just past the shoulders.

The boundaries of the four corners are the same as those of the immovable elbow. The eye-brows at the top, the groin area at the bottom and the area just past the shoulders on either side. The four corners are divided into four equal areas or gates.

For instance, the top half of the side of the lead hand is the outside high gate. Any attack to this gate will be blocked to the outside. Attacks to the inside gate will be blocked inward. Within each gate there are also two separate areas. A forward area and a rear area. Any attack to the forward area will be blocked by the forward arm. Attacks to the rear area will be handled by the hand that is back.

Practicing economy of movement in both defense and offense and keeping within the boundaries of the four corners is the heart of "Sil Lim Tao" (The 1st Wing Chun Form), (Little Imagination).

Any style which blocks and attacks simultaneously will be structurally faster than a style which incorporates a block and then an attack. The Chinese call the simultaneous block and attack, "Lin Sil Die Dar".

In Wing Chun footwork is essential so that you can face your opponent at all times. Since the structure of Wing Chun is based on straight punches, guarding the centerline, elbow-in, and immovable elbow, knowing how to face your opponent is essential.