Middle Term of Valid Categorical Syllogism is Distributed at least Once

Theorem
Let $M$ be the middle term of a valid categorical syllogism $Q$.

Then $M$ is distributed in at least one of the premises of $Q$ in which it appears.

Also see
To violate this rule is to commit the Fallacy of Undistributed Middle.


 * Rules of Quantity
 * Distributed Term of Conclusion of Valid Categorical Syllogism is Distributed in Premise

Historical Note
This rule of quantity was axiomatic at the time of, when the theory of the categorical syllogism was initially developed.