The disclosed generative composition system produces a composition to a briefing that describes a musical journey in emotional descriptions. The composition is assembled from concatenated interchangeable Form Atoms “FAs” selectable by tags aligning emotional descriptions with respective compositional heuristics. Each FA has self-contained constructional properties representative of an historical musical corpus. These heuristics support generation of chords, in chord schemes of musical tonics, achieving an equivalent form function. Each FA also includes chord spacer heuristics that temporally space generated chords across a defined musical window, and a chord list in a local tonic defining branching structures giving options for generating different chords. A progression descriptor, in combination with a form function, expresses musically a question, an answer or a statement, with each FA creating a meta-map of a chord scheme for a musical section. Musical transitions between FA reflect groupings in which FA have similar tags but different constructional properties.