I is the tonic, ii is the supertonic, iii is the mediant, IV is the subdominant, V is the dominant...so we were taught in school. This goes back to the work of Gottfried Weber (1779-1839), who gave the definitive form to scale-degree theories. Weber took the major and the minor scales to set up triads and seventh chords on each melodic degree and described those degrees through the use of large roman numerals for major triads (I, IV and V, in the major scale; V and VI in the minor), small roman numerals for minor triads and degree signs for the diminished chords. In the end, there were three types of triads (minor, major, diminished) and four types of seventh chords (minor, major, dominant, half-diminished). The other chords could be derived from these seven fundamental types, with a few exceptions - such as ninth chords. (to be continued...)