Neapolitan and augmented-sixth chords use fixed symbolic labels, not the generic degree+quality+inversion formula

Certain altered chords are conventionally labeled with fixed symbolic abbreviations rather than a derived scale-degree numeral: the Neapolitan sixth as N6 (or ♭II6), and the augmented-sixth chords as It+6, Fr+6, Ger+6 (Italian/French/German). These labels are drawn from a small closed set, not computed from the generic degree+quality+inversion formula used for ordinary diatonic and applied chords.

A Neapolitan chord (♭II in first inversion, spelled with the lowered second scale degree) is labeled with the generic derived formula as '♭ii6', or the label is typed inconsistently as 'Neapolitan,' 'N6,' 'NC,' etc. across different lessons.

Aldwell and Schachter (standard pedagogical labels for Neapolitan and augmented-sixth chords); Piston, Harmony. No Gould/Read/Ross equivalent — the same authority gap already noted for roman-numeral-case-convention.

This rule can be decided from pitches, durations and voices — no layout required. A validator can check it before anything is drawn.