Triads of the three minor scales — progression materials
To get you started in the direction of things that lay beyond the Major scale’s triad progression materials; here are two summary charts of the natural triads of the Major and three variants of minor scale: Natural minor, Harmonic minor, and Melodic minor. These charts catalog all differing triad-types and differing roots among these four scales.
Scale letter spellings are rendered in parallel C, but all other data is generic (applies to any key tonic). These triads are arrived at by using the “chord construction in thirds” technique discussed on the chord construction page. These are the greater pallets of triads from which the smaller chord progressions can be isolated.
Ultimately (in practice), the materials of all three variants of minor scale can be combined to avail the greater resource of what could be called THE minor mode. See bottom of Figure 1 “THE minor mode” (triads only are shown here).
A note about Roman numerals and minor scales
An earlier Figure (for Major scales only) listed the formal names and Roman numerals of scale degrees. The scale degree names of the minor scales(s) tones coincide closely to the Major’s but as you can see (in this more inclusive drawing, Figure 3) there are some differences in names (and some additional ones) from the sixth scale degree (Submediant) of minor up.
The sharp-sixth and sharp-seventh degrees of the minor scale, occur in the Harmonic and Melodic minor scales (better to think of them as raised-sixth and raised-seventh ). It might be confusing at first but raising the sixth and seventh degrees of minor (which may actually mean removing a flat) actually makes the tones coincide exactly with the Major scale’s natural sixth and seventh degrees. So, for example, the only difference between any melodic minor scale and it’s parallel Major is it’s third (being a flat-third). Their sixths and sevenths, and everything else other than the third are identical. Also note, the name Leading Tone applies to VII of Major but #VII of minor. Again, VII of Major and #VII of minor both refer to the exact same tone residing a Major-seventh interval (11°) above the tonic or one half-step (1°) below.
There are at least two conventions or standard ways of indicating triads with Roman numerals: all upper-case generic Roman, or the more descriptive upper and lower case plus extra symbols for diminished and Augmented (° and + respectively). Additionally, the raised 6th and 7th degrees of minor could (and should) be indicated with a # sign to distinguish them (their roots) from the roots of natural minor triads. In other words, you should see this (#vii°) for the VII triads of harmonic and melodic minor, but this (VII) for the VII triad of natural minor. Likewise, you should see this (#vi°) for the VI triad of melodic minor. You can’t count on everyone (or all books and all web sites) using that most precise notation however. So look for your clues whenever you see Roman numerals used for minor scales and progression chords. Try to ascertain (and understand) what method (or combination of methods) is being used.