At a low timescale (i.e. typically below 1 s) a music piece can be viewed as a combination of musical elements drawn from a limited inventory of predetermined conventional unitary items such as notes, duration values or chords. However, above a certain timescale, the organization of music is more relevantly described in terms of complex piece-specific objects linked to each other by a compact set of local self-deducible relationships, thus forming structured musical segments. The present report proposes a model called "system & contrast" (S&C), which aims at describing the inner organization of structural segments in terms of a carrier system (i.e. a set of elements forming a simple network of logical relationships) and a contrast, namely a substitutive element (usually the last one) which partly deviates from the logical sequence induced by the carrier system. We show that the S&C model is polymorphous in the sense that it applies to any type of musical information layer (melody, harmony, rhythm, rhymes, effects, etc...) in a very versatile way, therefore offering a powerful meta-description of musical content. The model has been designed for specifying a methodology for music structure annotation, but we briefly mention also its potential implications in other domains of music analysis and music information retrieval (MIR).