English: Schematic of a liquid crystal display element:

With the field turned off, the mesogens naturally form a twisted state between the crossed polarizers. Because of this, the polarization of incident light is rotated, and thus can pass through the assembly.

With the field turned on, the mesogens align between the crossed polarizers. Because of this, the light polarized at the upper polarizer cannot pass through the bottom polarizer. The entire assembly thus looks dark.