On a microscopic level there is little or no difference between crystalline and polycrystalline substances. Polycrystalline materials consist of small regions of crystalline material, oriented randomly. Within these microscopic regions the material can be perfectly crystalline. Typical sizes of the crystalline regions vary widely, but are usually found in the range of several micrometers. Although this size is very small, it corresponds to many lattice periods of the crystalline structure. In other words, within one small crystalline piece there would be many repititions of the regular crystal structural pattern.
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