This document discusses voids in different crystal structures including face-centered cubic (FCC), body-centered cubic (BCC), and hexagonal close-packed (HCP). It explains that voids exist between atoms due to incomplete packing. In FCC crystals, the voids take the shape of tetrahedra and octahedra. BCC crystals contain distorted tetrahedral and distorted octahedral voids. HCP crystals contain tetrahedral and octahedral voids identical to FCC. The size and positions of voids in each structure are analyzed. Carbon is shown to prefer the smaller octahedral void in BCC iron over the larger tetrahedral void, affecting its solubility.