Applied to a crystal system where the Bravais lattices have three sets of edges of unequal length, two sets of which are not at right angles to one another. They can therefore be referred to three unequal crystallographic axes a, b, and c (or x, y, and z). The a (or x) axis (clino) is inclined and forms an obtuse angle with the vertical c (or z) axis usually denoted by the symbol β‎. The b (or y) axis (ortho) is horizontal and at right angles to both the a (x) and c (z) axes.

Monoclinic