ABSTRACT
A single crystal can be thought of as composed of a regular repetition of an elementary building block known as the unit cell in all three dimensions. On the other hand, a polycrystal is a collection of single crystals with their crystalline axes oriented in different directions. If each of these single crystals constitute a grain, the boundaries between them will of course be synonymous with grain boundaries. Alternatively, a grain can encompass a number of single crystals.
