What is the tetragonal crystal system ?

The quadratic (or tetragonal) system is a crystalline system in which the unit cell of the lattice is a right prism with a square base (a=b). The characteristic symmetry element is an axis of order 4. This system includes 7 crystal classes. About 12% of minerals belong to this system, including chalcopyrite, scheelite, rutile, cassiterite, zircon, etc...