Process music is music that arises from a process. It may make that process audible to the listener, or the process may be concealed. Primarily begun in the 1960s, diverse composers have employed divergent methods and styles of process. "A 'musical process' as Christensen defines it is a highly complex dynamic phenomenon involving audible structures that evolve in the course of the musical performance ... 2nd order audible developments, i.e., audible developments within audible developments". These processes may involve specific systems of choosing and arranging notes through pitch and time, often involving a long term change with a limited amount of musical material, or transformations of musical events that are already relatively complex in themselves.