Cross product
The vector  is perpendicular to both  and . The direction of  is determined by a variation of the right-hand rule.  By holding your right hand as shown in the picture, your thumb points in the direction of  (the cross product of  and ), with the index finger pointing in the direction that  points, and the middle finger pointing in the direction that  points.  If the angle between the index (your pointer finger) and middle (second finger away from your thumb) fingers is greater than 180°, then you need to turn the hand upside down.