One extends to indefinite terms the Classical Syllogistics and the Rules of Valid Syllogism (RofVS). The valid syllo-gisms are generalized to conclusive syllogisms, made of conclusive pairs of categorical premises and their logical consequences (LCs), which all may contain indefinite terms: the positive, S,P,M, terms, and their complementary sets in the universe of discourse, U, i.e., the negative terms. A “pattern and type” classification, splits the 32 conclusive syllogisms into four types, Barbara, Darapti or Darii and Disamis – each containing eight syllogisms, which follow only three “patterns of inclusion and intersection”, namely either S⊆M⊆P (Barbara), or, M⊆S, M⊆P (Darapti), or M S≠Ø, M⊆P (Darii). (The Disamis type syllogisms follow the Darii pattern, but switch the roles played by the P and S terms.) By using only four Rules of Conclusive Syllogisms (RofCS), (and abandoning the RofVS “two negative prem-ises are not allowed” and “the middle term has to be dis-tributed in at least one premise”), one can make the RofCS into a “theory” "almost equivalent" with the formulas which describe all the conclusive syllogisms, including existential import syllogisms (to which the RofVS do not make any reference). Very importantly, each precise LC of the Barba-ra, Darapti and Darii patterns pinpoints a unique partitioning subset of U: S=S M P and P'=S' M' P' for Bar-bara’s pattern - which entails two LCs; M=M S P for Darapti’s pattern; S M P≠Ø for Darii’s pattern. The middle term’s elimination from the LC, initiated by Aristo-tle, simplifies, but weakens the LC.