A plane electromagnetic wave propagating along $x$ direction can have the following pairs of $\overrightarrow{\mathrm{E}}$ and $\overrightarrow{B}$.

As electric and magnetic field vectors $\overrightarrow{\mathit{E}}$ and $\overrightarrow{B}$ are perpendicular to each other as well as perpendicular to the direction of propagation of electromagnetic wave,

 ${\hat{\mathrm{E}}}_{\mathrm{y}} \times {\hat{\mathrm{B}}}_{\mathrm{z}} = {\hat{\mathrm{v}}}_x$ 

similarly, ${\hat{\mathrm{E}}}_{\mathrm{z}} \times {\hat{\mathrm{B}}}_{\mathrm{y}} = -{\hat{\mathrm{v}}}_x$.