Page "Liquid crystal display" Paragraph 13

The optical effect of a twisted nematic device in the voltage-on state is far less dependent on variations in the device thickness than that in the voltage-off state.

Because of this, these devices are usually operated between crossed polarizers such that they appear bright with no voltage ( the eye is much more sensitive to variations in the dark state than the bright state ).

These devices can also be operated between parallel polarizers, in which case the bright and dark states are reversed.

The voltage-off dark state in this configuration appears blotchy, however, because of small variations of thickness across the device.