Electromagnetism is an example of a gauge theory. Here the Lie group is , the one-dimensional unitary group, which is in particular abelian. There are gauge theories, such as Yang–Mills theory, in which the Lie group is not abelian. In that case, one gets relations which are similar to those described here. The analog of the field in such theories is the curvature form of the connection, which is represented in a gauge by a Lie algebra-valued one-form . The Yang–Mills field is then defined by