Statistics from about 17,000 occurrences of the structures “N1 is N1” and “N1 is N7” have proved that (a) there is a functional difference between the two predicative cases and (b) there are strong norms for selecting one of the two cases in communication. The nominative case is a strong norm if the communicative function of the sentence predicate is to (a) identify a sort of denotation in a demonstrative act, (b) identify a sort of denotation in a nominative act, (c) define using qualification or (d) qualify in an expressive manner. The nominative is preferred when stating a person’s profession, in most cases (except for professions such as minister, director, manager etc., especially in a specific sentence structure and discourse function). The statistics show that (a) for 630 different predicate nouns (PN), only the nominative is used and for 270 PN, only the instrumental is used; (b) for 900 PN, one of the two predicative cases is used either exclusively or with a strong preference; (c) for only 89 PN, the use of the two cases is balanced. The corpus statistics for the two predicative cases show that the selection of one of these cases is semantically determined and to a great degree lexically bound.