If $\overline{\mathrm{a}}, \overline{\mathrm{b}}, \overline{\mathrm{c}}, \overline{\mathrm{d}}$ are any four vectors then $(\overline{\mathrm{a}}\times \overline{\mathrm{b}})\times (\overline{\mathrm{c}}\times \overline{\mathrm{d}})$ is a vector

$\begin{array}{rcl} & =& (\overline{\mathrm{a}}\times \overline{\mathrm{b}})\times (\overline{\mathrm{c}}\times \overline{\mathrm{d}})\\ & =& [\overline{\mathrm{a}} \overline{\mathrm{b}} \overline{\mathrm{d}}] \overline{\mathrm{c}}-[\overline{\mathrm{a}} \overline{\mathrm{b}} \overline{\mathrm{c}}] \overline{\mathrm{d}}\end{array}$

equally inclined $\overline{\mathrm{a}}\times \mathrm{b}$ and $\overline{\mathrm{c}}\times \overline{\mathrm{d}}$ and 

along the line of intersection of two planes one containing $\overline{\mathrm{a}}, \overline{\mathrm{b}}$ and other containing $\overline{\mathrm{c}}, \overline{\mathrm{d}}$