Quiz: Remember the definition of mal de mer?
one of the structural categories to which crystalline solids can be assigned. Crystals in this system are referred to three mutually perpendicular axes, two of which are equal in length. If the atoms or atom groups in the solid are represented by points and the points are connected, the resulting lattice will consist of an orderly stacking of blocks, or unit cells. The tetragonal unit cell is distinguished by an axis of fourfold symmetry, about which a rotation of the cell through an angle of 90 brings the atoms into coincidence with their initial positions. The elements boron and tin can crystallize in tetragonal form, as can some minerals such as zircon.