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Along with the tetrahedron, the octahedron is one of predominant geometrical units in inorganic structures. It is encountered commonly in salts (such as sodium chloride), molecules containing transition metals, and a variety of metallic structures. You can build one in a variety of ways. You can start with a square and put points above and below it. Alternatively, you can take two equilateral triangles and lower one over the other in parallel. Then rotate them by 60 degrees relative to each other until the distances between the corners of neighboring triangles equal those within the triangles. This polyhedron is another of the five Platonic Solids.