Musical form is one of the most central aspects of musical structure, as it concerns the overarching organization principles of music across genres and styles. Therefore, understanding the formal characterization of musical form is a central topic in music theory, computational music analysis, MIR, and music generation. This paper makes a theoretical contribution proposing a formal model that characterizes the main aspects of musical form. We characterize musical form by the following aspects: Grouping structure, rhythmic partitioning, formal functions, repetition structure, schemata, and harmonic anchor points. As the structures of hierarchical segmentation as well as formfunctionality have previously been conceptualized in terms of a recursive tree-shaped hierarchy, we ground our model in abstract generative grammars. Our model extends this hierarchical analysis by an account of the rhythmical properties of form as well as repetition structure. The harmonic layout defines constraints for motivic content (pitch and rhythm). Our approach also addresses repetition structure by modeling the location and degree of variation of repeated ideas. This is achieved via variable binding. We exemplify our theoretical contribution by a detailed analysis.