How this instrument works
Every major key has seven notes, and stacking a triad on each one produces seven diatonic chords with a fixed, predictable pattern of qualities: the 1st, 4th and 5th scale degrees (I, IV, V) form major chords, the 2nd, 3rd and 6th degrees (ii, iii, vi) form minor chords, and the 7th degree (vii°) forms a diminished chord. Roman numerals — capital for major, lowercase for minor, with a small circle for diminished — are the standard shorthand musicians use to describe a chord's position and quality within a key, without tying it to one specific key's letter names.
A 'progression' is just a sequence of these diatonic chords played in a particular order, and certain sequences have become so common they're named and taught as standard patterns: I-IV-V-I is the basic authentic-cadence progression that ends countless pieces of Western music, I-V-vi-IV is the so-called 'four chords' pattern behind an enormous number of pop songs, and ii-V-I is the standard turnaround that closes most jazz tunes. These aren't arbitrary — they exploit the specific pull each scale degree has toward resolution, especially the dominant (V) chord's strong tendency to resolve back to the tonic (I).
Pick any major key and this instrument transposes the same progression pattern onto that key's own diatonic chords — the I-IV-V-I pattern in C major (C-F-G-C) has the exact same underlying relationship as the I-IV-V-I pattern in G major (G-C-D-G), just shifted to different letter names. This uses the same major-key diatonic-triad logic as any music theory textbook; minor-key progressions are a separate topic not covered here.
- Choose a Key (major) — the major key whose diatonic chords the progression will be built from.
- Choose a Progression preset — a named, commonly-taught sequence like I-IV-V-I or ii-V-I.
- Read Chord progression for the actual chord names, in order, spelled out for your chosen key.
- Try the same progression preset across different keys to see how the pattern transposes while keeping its harmonic relationships intact.
- Compare different progression presets in the same key to hear how each pattern creates a different harmonic feel.
Worked example — I-IV-V-I in the key of C major
C major's scale is C, D, E, F, G, A, B — 7 notes in total — so its 1st, 4th and 5th scale degrees (Roman numerals I, IV, V) are C, F and G respectively. Since I, IV and V are always major-quality chords in a major key, the I-IV-V-I progression spells out as C - F - G - C — the classic authentic-cadence pattern that resolves cleanly back to its starting chord.
Apply that identical I-IV-V-I pattern to G major instead (scale G, A, B, C, D, E, F#, again 7 notes) and the same relationship transposes to G - C - D - G — different letter names, but the exact same functional relationship between tonic, subdominant and dominant chords. That's the whole point of Roman-numeral analysis: it describes the relationship, not the specific pitches.
Questions
What do the Roman numerals I, IV, V, ii, vi mean?
They label a chord by which scale degree of the key it's built on and, through capitalization, its quality: capital numerals (I, IV, V) are major chords, lowercase (ii, iii, vi) are minor chords, and a lowercase numeral with a small circle (vii°) is diminished. This notation lets musicians describe a chord progression's structure independent of any specific key — 'I-IV-V-I' means the same relationship whether you're in C major or G major.
Why is I-V-vi-IV called the 'four chords' progression?
It earned that nickname because an enormous number of well-known pop songs across many decades and genres use this exact same four-chord sequence, sometimes starting at a different point in the cycle (vi-IV-I-V is the same four chords, just reordered to start elsewhere) — it's become something of a running joke and popular internet meme among musicians precisely because of how frequently it's reused. This instrument includes both the I-V-vi-IV ordering and the vi-IV-I-V rotation as separate presets.
What makes ii-V-I the 'standard jazz turnaround'?
It creates a strong, smooth harmonic pull toward resolution: the ii chord sets up tension, the V (dominant) chord intensifies it, and the I (tonic) chord resolves it — jazz musicians use this three-chord sequence constantly, both as a way to end a tune and as a recurring building block within longer chord progressions, often substituting more complex chord extensions over the same underlying ii-V-I skeleton.
Does this work for minor keys too?
No — this instrument's progression presets and diatonic-quality table are built specifically for major keys, where the pattern of major/minor/diminished chords across the seven scale degrees follows the standard major-scale structure. Minor keys have their own, different diatonic chord-quality pattern (and several minor-scale variants), which is a separate topic outside this instrument's current scope.
Can I use these progressions in any musical style?
Yes — these are foundational, style-agnostic patterns that show up across classical, pop, rock, folk, and jazz music, though different genres tend to favor different presets (I-IV-V-I is common in rock and folk cadences, ii-V-I dominates jazz, I-V-vi-IV is ubiquitous in mainstream pop). The chords this instrument generates are correct for any style; how you voice, rhythm, and arrange them is where genre character comes from.