* The union of two transitive relations need not be transitive. For instance, "was born before or has the same first name as" is not a transitive relation, since e.g. [[Herbert Hoover]] is related to [[Franklin D. Roosevelt]], which is in turn related to [[Franklin Pierce]], while Hoover is not related to Franklin Pierce. | * The union of two transitive relations need not be transitive. For instance, "was born before or has the same first name as" is not a transitive relation, since e.g. [[Herbert Hoover]] is related to [[Franklin D. Roosevelt]], which is in turn related to [[Franklin Pierce]], while Hoover is not related to Franklin Pierce. |