A substance that flows like a liquid but has some order in its arrangement of molecules. Nematic crystals have long molecules all aligned in the same direction, but otherwise randomly arranged. Cholesteric and smectic liquid crystals also have aligned molecules, which are arranged in distinct layers. In cholesteric crystals, the axes of the molecules are parallel to the plane of the layers; in smectic crystals they are perpendicular. The names for the different types of liquid crystals were proposed by the French crystallographer Georges Friedel, who studied such crystals extensively in the early 20th century. Many of the key concepts used in other branches of the theory of condensed matter, such as order and symmetry, are used in the theory of liquid crystals. Applications include the liquid-crystal displays used in television and computer screens.