The attribute "normal" is a vector normal (perpendicular) to the chord of the arc, the line joining the first and last point of the arc. In a 2D coordinate system, there are only two possible directions for the normal, and it is often given as a signed real, indicating its length, with a positive sign indicating a left turn angle from the chord line, and a negative sign indicating a right turn from the chord. In 3D, the normal determines the plane of the arc, along with the start and endPoint of the arc. The normal is usually a unit vector, but this is not absolutely necessary. If the normal is a zero vector, the geometric object becomes equivalent to the straight line between the two end points. The length of the normal sequence is exactly the same as for the bulge sequence, 1 less than the control point sequence length.