A crystalline solid has as a fundamental property the ordered structure of its atoms. This structure consists of an array of sites forming a regular three-dimensional periodic lattice. The ordered structure, strictly speaking, does not mean that the atoms are located exactly on the sites of the lattice as they are in constant motion due to thermal agitation. In fact, the average positions of the atoms are the points in space that should be considered as the sites of the ordered lattice. Only for simplicity we say that atoms themselves form the crystal lattice.