A large number of chemical compounds are known to exhibit one or several liquid crystalline phases. Despite significant differences in chemical composition, these molecules have some common features in chemical and physical properties. There are three types of thermotropic liquid crystals: discotics, bowlics and rod-shaped molecules. Discotics are flat disc-like molecules consisting of a core of adjacent aromatic rings; the core in a bowlic is not flat but like a rice bowl (a three-dimensional object).   This allows for two dimensional columnar ordering, for both discotics and bowlics. Rod-shaped molecules have an elongated, anisotropic geometry which allows for preferential alignment along one spatial direction.