This article introduces a model called “System & Contrast” (S&C), which aims at describing the inner organization of structural segments within music pieces in terms of : (i) a carrier system, i.e. a sequence of morphological elements forming a multi-dimensional network of self-deducible syntagmatic relationships and (ii) a contrast, i.e. a substitutive element, usually the last one, which partly departs from the logic implied by the rest of the system. With a primary focus on pop music, the S&C model provides a framework to describe internal implication patterns in musical segments by encoding similarities and relations between its constitutive elements so as to minimize the complexity of the resulting description. It is applicable at several timescales and to a wide variety of musical dimensions in a polymorphous way, therefore offering an attractive meta-description of different types of musical contents. It has been used as a central component in the creation of a set of annotations for 380 pop songs (Bimbot, Sargent, Deruty, Guichaoua & Vincent, 2014). This article formalizes the S&C model, illustrates how it applies to music and establishes its filiation with Narmour’s Implication-Realization model (Narmour 1990, 1992) and Cognitive Rule-Mapping (Narmour, 2000). It introduces the Minimum Description Length scheme as a productive paradigm to support the estimation of S&C descriptions and sketches several tracks where concepts from the domain of Electrical Engineering and Communication Systems can be paralleled with aspects pertaining to the structural description of music patterns by the S&C model. Strongly based on an Engineering Science viewpoint, the S&C model establishes promising connections between Music Data Processing and Information Retrieval on the one hand, and modern theories in Music Perception and Cognition on the other hand, together with interesting perspectives in other areas in Musicology.