A generalization of the concept of space group to include magnetic ordering. In such groups, the degeneracy (see degenerate level) associated with the Kramers theorem is lifted. There are 122 point groups and 1651 space groups for magnetic crystal symmetry. These groups are also sometimes referred to as colour groups or Shubnikov groups, in honour of Aleksei Vasilevich Shubnikov (1887–1970), who worked out the theory of such groups. Some difficulties arise in the description of electronic states with magnetic crystal groups that do not arise in the case of ordinary space groups; however, these difficulties can be dealt with.