Let's take a closer look at the structure of an actual LCD.

The two panels sandwiching the liquid crystals are grooved. They are formed so that the liquid crystal molecules line up along the grooves. The liquid crystals are sandwiched by arranging these two panels with their grooves crossed at right angles, causing each layer of liquid crystals molecules to gradually twist until the top layer is at a 90-degree angle to the bottom one.

The axis of the light vibration that has passed through the polarizing filter is also twisted 90 degrees as it travels through these liquid crystals. Since the alignment direction of liquid crystal molecules depends on voltage, applying voltage causes them to line up along the electric field and untwist, also allowing light to be untwisted. The light that passes through the first polarizing filter will be blocked by the second polarizing filter and will not go through.

This is the basic principle of a twisted nematics (TN) LCD. In STN LCDs, the angle of twist is 120 degrees or more.