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Metals are polycrystals made up of many crystal lattices. These grids, clusters of atoms, provide in a sense the basic structure of a metal or an alloy. Atoms are arranged in specific standard patterns. Iron (Fe), for example, forms cubes at room temperature (see figure), with atoms sitting in the corners and at the center of each cube. For this reason, we say that iron has a body-centered cubic (bcc) lattice. Iron with such a lattice structure is called α - iron or ferrite. As temperature increases, it assumes a face-centered cubic lattice structure and it is then called austenite. The current state of science cannot explain why iron has a bcc structure at room temperature. See also Lattice defect.