The vector formula_6 is perpendicular to both formula_7 and formula_8. The direction of formula_6 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 formula_4 (the cross product of formula_7 and formula_8), with the index finger pointing in the direction that formula_7 points, and the middle finger pointing in the direction that formula_8 points. If the angle between the index and middle fingers is greater than 180°, then you need to turn the hand upside down.