Relational, operator, and predicate systems are distinguished on the basis that they correspond to the three possible pair-wise bracketings into two constituents of the three parts of a proposition: relation, subject, and object. It is asserted that the verbal propositional modality (left hemisphere) uses a predicate grammar, while the spatial-image operational modality (right hemisphere) uses an operator grammar. Verbal propositional memory has the capacity for extensive propositional embedding while spatial operational memory does not.