Norton's theorem is similar to Thevenin's theorem. The difference is that the Norton-equivalent circuit has a constant-current source and the impedance is in parallel with this source, rather than in series. The above circuit diagram is the Norton equivalent of the same circuit used to illustrate Thevenin's theorem above. It is important to have a cursory knowledge of Thevenin's theorem because it relates to understanding the output impedance of a circuit. Norton's theorem is generally useful only from a theoretical viewpoint and will not be discussed further here.