# Fallacy of the undistributed middle | | The **Fallacy of the undistributed middle** is a formal fallacy that is committed when the middle term in a categorical syllogism is not distributed in either the minor premise or the major premise. It is thus a syllogistic fallacy. | |-|-| | | wikipedia:: [Fallacy of the undistributed middle](https://en.wikipedia.org/wiki/Fallacy_of_the_undistributed_middle) | > The fallacy of the undistributed middle occurs when the term that links the two premises is never distributed. > > In this example, distribution is marked in boldface: > > All Z is B > All Y is B > Therefore, all Y is Z > B is the common term between the two premises (the middle term) but is never distributed, so this syllogism is invalid. B would be distributed by introducing a premise which states either All B is Z, or No B is Z. > > Also, a related rule of logic is that anything distributed in the conclusion must be distributed in at least one premise. > > All Z is B > Some Y is Z > Therefore, all Y is B > The middle term—Z—is distributed, but Y is distributed in the conclusion and not in any premise, so this syllogism is invalid.