A mule note is a bank note that is made using back plates from a different series of notes. Most mule notes have different size plate numbers on the front and back.
The most common way to make a mule note is by using the back plates from a previous series of notes. When the transition to macro plates originally took place, micro and macro plates were used side by side, resulting in some notes being made using mismatched plates. In some cases, entire series of notes were made using this method, resulting in the non-mule note being scarcer than the mule.