The notation and terminology of this paper follow [2], and are dual to those of [6] and [7]. If L is a language in the narrow sense, Cn may be any consequence operation on sets of sentences of L that includes classical sentential logic. Henceforth when we talk of the language L we intend to include reference to some fixed, though unspecified, operation Cn. X is a deductive system if X = Cn(X). Sentences x, z that are logically equivalent with respect to Cn – that is x ∈ Cn({z}) and z ∈ Cn({x}) – are identified. If X and Z are systems we often write X x instead of x ∈ X and Z X instead of X ⊆ Z. If X = Cn({x}) for some sentence x, X is (finitely) axiomatizable. The set theoretical intersection of X and Z has the logical force of disjunction, and is written X ∨ Z; Cn(X ∪ Z), the smallest system to include both X and Z, is written XZ. If K is a family of systems, [K] and [K] may be defined in an analogous way. The logically strongest system S is the set of all sentences of L; the weakest system T is defined as Cn(∅). The autocomplement Z of Z is defined to be the strongest system to complement T, namely the system [{Y : Y ∨ Z = T}]. More generally we may define X − Z as [{Y : X Y ∨ Z}]. In terms of this operation to remainder, Z is identical with T − Z. The class of all deductive systems forms a distributive lattice under the operations of concatenation and ∨; and indeed a Brouverian algebra (a relatively authocomplemented lattice with unit) under concatenation, ∨ – and T.