Chords in a Key

The Chords concept named chords in isolation. But a chord also has a job inside a key. Stack a third on each note of a scale and you get the diatonic chords - the seven that belong to the key - each labelled with a Roman numeral (I, ii, iii…) for its degree and quality. Pick a key and press them; play your own chord and the readout tells you which numeral it is, or flags it as an outsider.

Upper-case numerals are major (I, IV, V), lower-case are minor (ii, iii, vi), and ° marks the diminished chord (vii°). The numeral is relative to the key: the V of C major is G, the V of D major is A - same function, different chord.

Why V wants to go home

The V (the dominant) is the engine of tonal music: it contains the leading tone, a half-step below the tonic, which pulls strongly back to I. Toggle stack sevenths and V becomes V7 - even hungrier to resolve. The V7 → I cadence is the most important progression you'll meet.

The minor-key twist

In a natural-minor key the chord on the fifth degree is minor (a weak v) - so minor-key music almost always borrows the raised 7th from the harmonic minor to make a major V (and a vii°), restoring that pull home. Pick a minor key above and you'll see both: the honest diatonic v, and the real-world dominant shown separately.

Three jobs: tonic, subdominant, dominant

The seven chords share just three roles. Tonic chords (on degrees 1, 3 and 6) are stable and establish the key; they are where tension comes to rest. Subdominant chords (degrees 2 and 4) lead the music away from home. Dominant chords (degrees 5 and 7) are the most tense and point straight back to the tonic. A move from a non-tonic chord home is a cadence; the strongest of all, V to I, is the perfect cadence at the heart of Western music.

Progressions you already know

String these roles together and you have a chord progression. A handful turn up everywhere: I-IV-V is the backbone of blues and rock, I-V-vi-IV powers countless pop songs, and ii-V-I is the engine of jazz. Because the Roman numerals are relative to the key, the same progression transposes to any key just by reading off that key's diatonic chords.

Diatonic vs borrowed

Play a chord that isn't one of the seven and the readout calls it chromatic / borrowed - it's still a real chord (the Chords concept will name it), it just doesn't belong to this key. That tension between belonging and borrowing is where harmony gets its colour, and it's the doorway to everything beyond these concepts.