Although the distinction between software and hardware is a posteriori, there is an a priori distinction that masquerades as the software—hardware distinction. This is the distinction between procedure interconnection, the semantics of flow chart diagrams, which is known to be described by the regular expression calculus; and system interconnection, the semantics of network diagrams, which is described by a certain logical calculus, dual to a calculus of regular expressions. This paper presents a proof of the duality in a special case, and gives the interpretation of the logical calculus for sequential machine interconnection. A minimal realization theorem for feedback systems is proved, which specializes to known open loop minimal realization theorems.