The crystals in which the periodicity of the pattern extends throughout the piece of the crystal are known as single crystals. Single crystals have an isotropic behavior i.e. their physical properties (like mechanical strength, refractive index, thermal and electrical conductivity) are different along different directions. The small size single crystals are called mono-crystals.
A ply crystal is the aggregate of the mono crystals whose well developed faces are joined together so that it has isotropic properties. Ceramics are the important illustrations of the poly-crystalline solids.
The organic crystalline solid which on heating to a certain temperature range becomes fluid like but its molecules remain obtained in a particular directions, showing that they retain their anisotropic properties, is called liquid crystal. These crystals are used in a liquid crystal displays (L.C.D) which are commonly used in electronic watches, clocks and micro calculator etc.