Tetragonal crystal system

A crystal system with mutually perpendicular conventional cell axes and lengths $a=b\ne c$. The physical length of the direction $[uvw]$ is proportional to $\sqrt{a^2(u^2+v^2)+c^2{w}^2}$; crystallographic indices are coefficients in a lattice basis, not necessarily Cartesian components of equal scale.

A Bravais lattice with mutually perpendicular primitive vectors of lengths $a,a,c$, with $c\ne a$. A chemically ordered L10 crystal structure has this translation lattice even when drawn using a larger four-site cell that resembles a distorted FCC conventional cell.