In some crystalline materials the magnetic moments associated with certain atoms adopt long-range order. This magnetic ordering is described as a lattice of space-filling unit cells whose contents obey symmetry constraints. The magnetic unit cell does not necessarily match the unit cell, or unit-cell origin, of the nuclear or `parent' lattice. The relationship between the two lattices is summarized by propagation vectors, most often a single vector in reciprocal space describing the translational symmetry of the magnetic lattice expressed relative to the nuclear reciprocal-lattice unit-cell lengths. Magnetic ordering is classed as commensurate when all three components of the propagation vector are the reciprocals of integers; otherwise, the structure is incommensurate. Two complementary approaches to describing magnetic symmetry exist: (a) magnetic space groups or (b) the symmetry modes of the irreproducible representations of the parent space group (`irreps'). The magnetic CIF dictionary provides data names for describing a structure in terms of magnetic space and superspace groups only.