TN
The twisted nematic effect (TN-effect) is the main breakthrough that made liquid crystal displays practical. Unlike earlier displays, TN-cells did not require a current to flow for operation and used low operating voltages suitable for use with batteries. The introduction of TN-effect displays led to their rapid expansion in the display field, quickly pushing out other common technologies like light emitting diodes and electroluminescence from most electronics. By the 1990s, TN-effect LCDs were largely universal in portable electronics. This electro-optical effect is also called Schadt–Helfrich effect.

The twisted nematic effect is based on the precisely controlled realignment of liquid crystal molecules between different ordered molecular configurations under the action of an applied electric field. This is achieved with little power consumption and at low operating voltages.
Exploded view of a TN liquid crystal cell showing the states in an off state(left), and in an on state with voltage applied(right)

The illustrations to the right show both the OFF and the ON-state of a single picture element (pixel) of a twisted nematic lgght modulator. A twisted configuration of nematic liquid crystal molecules is formed between two glass plates, G, which are separated by several spacers and coated with transparent electrodes, E1, E2. The electrodes themselves are coated with alignment layers (not shown) that precisely twist the liquid crystal by 90° when no external field is present (left diagram). When light shines on the front of the LCD, light with the proper polarization (about half) will pass through the first polarizer and into the crystal, where it is rotated by the helical structure. The light is then properly polarized to pass through the second polarizer, set at 90° to the first. The light then passes through the back of the cell, which thus looks transparent.
When a field is applied between the two electrodes, the crystal re-aligns itself with the external field (right diagram). This "breaks" the careful twist in the crystal and fails to re-orient the polarized light passing through the crystal. In this case the light is blocked by the rear polarizer, and the cell becomes opaque.

The amount of opacity can be controlled by varying the voltage; at voltages near the threshold only some of the crystals will re-align, and the display will be partially transparent, but as the voltage is increased more and more of the crystals will re-align until it becomes completely "switched". A voltage of about 1 V is required to make the crystal align itself with the field, and no current passes through the crystal itself. Thus the electrical power required for that action is very low.

To display information with a twisted nematic liquid crystal, the transparent electrodes are structured by photo-lithography to form a matrix or other pattern of electrodes. Only one of the electrodes has to be patterned in this way, the other can remain continuous (common electrode). For low information content numerical and alpha-numerical TN-LCDs, like digital watches or calculators, segmented electrodes are sufficient. If more complex data or graphics information have to be displayed, a matrix arrangement of electrodes is used. Obviously, addressing of matrix displays, such as in LCD-screens for computer-monitors or flat television screens, is more complex than with segmented electrodes. These matrix LCDs necessitate integration of additional non-linear electronic elements into each picture element of the display (e.g. thin-film diodes, TFD s, or thin-film transistors, TFTS) in order to allow the addressing of individual picture elements without crosstalk (unintended activation of non-addressed pixels).