This article introduces the System & Contrast (S&C) model, which aims at describing the inner organization of structural segments within music pieces as: (i) a carrier system , i.e., a sequence of morphological elements forming a network of self-deducible syntagmatic relationships, and (ii) a contrast , i.e., a substitutive element, usually the last one, which departs from the logic implied by the carrier system. Initially used for the structural annotation of pop songs (Bimbot, Deruty, Sargent, & Vincent, 2012), the S&C model provides a framework to describe implication patterns in musical segments by encoding similarities and relations between its elements. It is applicable at several timescales to various musical dimensions in a polymorphous way, thus offering an attractive meta-description of musical contents. We formalize the S&C model, illustrate how it applies to music and establish its filiation with Narmour’s implication-realization model (Narmour, 1990, 1992) and cognitive rule-mapping (Narmour, 2000). We introduce the minimum description length scheme as a productive paradigm to support the estimation of S&C descriptions. The S&C model highlights promising connections between music data processing and information retrieval on the one hand, and modern theories in music perception, cognition and semiotics on the other hand, together with interesting perspectives in Musicology.