A set of reference axes used in crystallography. These axes are usually three in number, although in some cases, they may be four. The axes are generally mutually perpendicular, coincide with symmetry axes or the normals to symmetry planes, and in cases where the crystal lacks symmetry, parallel to lines of intersection of two faces with greatest areas. These axes are designated as a, b, c, and angles between axes are designated α, β, and γ where α is located between axes b and c, β is located between axes a and c, etc. according to the right- hand rule.
See also crystal system.