A normal modal logic "L" corresponds to a class of frames "C", if "C" = Mod("L"). In other words, "C" is the largest class of frames such that "L" is sound wrt "C". It follows that "L" is Kripke complete if and only if it is complete of its corresponding class. Consider the schema T : formula_20. T is valid in any reflexive frame formula_1: if formula_22, then formula_13 since "w" "R" "w". On the other hand, a frame which validates T has to be reflexive: fix "w" ∈ "W", and define satisfaction of a propositional variable "p" as follows: formula_24 if and only if "w" "R" "u". Then formula_25, thus formula_26 by T, which means "w" "R" "w" using the definition of formula_4. T corresponds to the class of reflexive Kripke frames. It is often much easier to characterize the corresponding class of "L" than to prove its completeness, thus correspondence serves as a guide to completeness proofs. Correspondence is also used to show "incompleteness" of modal logics: suppose "L"1 ⊆ "L"2 are normal modal logics that correspond to the same class of frames, but "L"1 does not prove all theorems of "L"2. Then "L"1 is Kripke incomplete. For example, the schema formula_28 generates an incomplete logic, as it corresponds to the same class of frames as GL (viz. transitive and converse well-founded frames), but does not prove the GL-tautology formula_29. For any normal modal logic "L", a Kripke model (called the canonical model) can be constructed, which validates precisely the theorems of "L", by an adaptation of the standard technique of using maximal consistent sets as models. Canonical Kripke models play a role similar to the Lindenbaum–Tarski algebra construction in algebraic semantics. A set of formulas is "L"-"consistent" if no contradiction can be derived from them using the axioms of "L", and modus ponens. A "maximal L-consistent set" (an "L"-"MCS" for short) is an "L"-consistent set which has no proper "L"-consistent superset.