NEPTUNE is a harmonic mapping tool developed within Harmogénèse, a personal research framework by Jonathan Marty-Wagner dedicated to Bitonal Synesthesia, extended modal systems, harmonic color, and the relationships between chords, scales, tensions, and musical perception. The tool allows the user to select a scale family, a mode, and a transposition root, then observe which chord structures can exist within that modal space. NEPTUNE is not designed merely to display the notes of a scale, but to reveal a harmonic territory: available chords, internal tensions, modal colors, and areas of stability, ambiguity, or instability. This first version includes several modal families, including the major scale, melodic minor, harmonic minor, harmonic major, and double harmonic major. It also includes an option for exploring altered dominant chord families. NEPTUNE belongs to a research-creation approach: building musical thinking instruments that connect theory, composition, sensation, and harmonic exploration. French note: NEPTUNE est un atlas d’accords et de modes étendus développé dans le cadre d’Harmogénèse. Il permet de cartographier les accords contenus dans un espace modal donné, afin de penser l’harmonie comme un territoire vivant plutôt que comme une simple liste de formules.