I'm reading about stazionary waves in a cube with side of lenght L. The electric field is described from a vector $E$ where each component is of the type $sin(\frac{\pi}{L}n_{j}x_{j})$ and the author says that if I call $K=\frac{\pi}{L}(n_{1},n_{2},n_{3})$, where $n_{j}$ is natural, I have two polarizations for each $K$, "one for each orthogonal direction' Why?

I'm reading about stationary waves in a cube with side of length $L$. The electric field is described from a vector $E$ where each component is of the type $sin(\frac{\pi}{L}n_{j}x_{j})$ and the author says that if I call $K=\frac{\pi}{L}(n_{1},n_{2},n_{3})$, where $n_{j}$ is natural, I have two polarizations for each $k$, "one for each orthogonal direction' Why?