Right-hand rule

The right-hand rule is a rule that appears in the cross product,

\mathbf{e}_z = \mathbf{e}_x \times \mathbf{e}_y. $$ The direction of the unit vector ez given as the cross product of unit vectors along the x- and y-axis is given by the right-hand rule, see photograph.

The case depicted here has a right (90° degree) angle &alpha; between the x- and the y-axis. However, the rule works for any angle &alpha; between x- and y-axis, as long as  0° < &alpha; < 180°. The z-axis is always perpendicular to the plane containing the x- and y-axes. If the angle &alpha; equals 0° or 180°, the cross product is the zero vector, which has an undetermined direction.