# Polarisation directions in standing waves in cubical cavity

I was studying Rayleigh-Jean's formula. The author has assumed a cubical cavity of each side $L$ with perfectly reflecting surfaces. According to author, there are two perpendicular directions of polarization for a standing wave taken in arbitrary direction. I am stuck up here, my question is why two directions of polarization?

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It is possible because there are two linearly-independent solutions for wave-equation of the light traveling in the direction $\vec n$. Let $\vec n$ coincides with z axis then there are two polarizations for $\vec E$: x- and y-. Every wave of light traveling along z axis may be seen as superposition of two waves with x- and y- polarizations.