A: This right-handed spiral rule is often used for vector cross product. See the figure below, which vector is the first multiplicand when the vector is cross-product, that is, the projection of the vector on the xOy plane, the angle formed by counterclockwise rotation with the positive direction of the X axis as the initial angle of 0 degrees, the vector with the smallest angle as the multiplicand, multiplied by the vector with the larger angle, the Z direction value of the plane normal vector must be positive, otherwise it will be negative. In plan view, OA angle is the smallest and OC angle is the largest.

In terms of gestures, the right hand is placed on the xOy plane, the four fingers bend from the positive direction of the X axis to the positive direction of the Y axis, and the thumb points in the direction of the Z axis. Seen from the plan, the red curve is the direction of four fingers, and the thumb points to the paper. In fact, the four-finger direction is the vector order direction of the vector cross product. From the three vectors in the graph: OAxOB, OBxOC and OAxOC, the z value of the vector obtained must be positive; Which is the positive vector of the plane. The order of cross product is the order of I, II, III and IV of the four quadrants.