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.