Adding chords: I, IV, V and vi

With four chords, C, F, G and Am, every note of the C major scale belongs to at least one of them.

A melody can stand alone, but most songs put chords underneath it. A chord here means three notes sounded together, built by stacking every other note of the scale: a note, the note two steps above it, and the note two steps above that. Musicians call this three-note stack a triad, and the bottom note is its root. In C major you can build a triad on each of the seven scale notes.

Roman numerals

Chords are named in two ways. A chord symbol such as C or Am names the root and says whether the chord is major or minor. A Roman numeral names the chord by its place in the key: the chord built on the first note is I, on the fourth note IV, on the fifth V, on the sixth vi. Uppercase numerals mean major chords and lowercase numerals mean minor chords. In C major the seven triads are I, ii, iii, IV, V, vi and vii°, where the small circle marks a diminished chord. The numerals stay the same in every major key, which is why songwriters use them to talk about progressions.

Why these four

I, IV and V are called the primary chords. I is the tonic, the chord of home and rest. V is the dominant; it contains B, the note just below the home C, and it pulls strongly back toward I. IV is the subdominant, which moves away from home without the strong pull of V. Together these three chords contain all seven notes of the scale, so any melody note can be harmonised by one of them. The vi chord, A minor, adds a darker colour. It shares two notes, C and E, with the I chord, so it can often stand in for I when you want a change of mood.

Choosing a chord for a bar

To pick a chord, look at the notes in the bar, especially the ones on beat 1 and the long ones. Choose the chord that contains most of them. In the grid, notes that belong to their bar's chord are drawn solid and notes outside it are pale. A pale note is not wrong: short notes that pass between chord tones are normal in almost every melody. Theory books call them non-chord tones. What sounds unsettled is a long note or a beat-1 note that clashes with the chord, so fix those first. The panel shows, for each bar, how many of its notes are chord tones.

Some orders of chords are so common that they have become habits of pop and folk songs. I, IV, V, I is the oldest pattern in this course. I, vi, IV, V and I, V, vi, IV both use all four chords and loop easily. Ending a section with V then I gives a strong full stop, which the question and answer unit uses on purpose.

Try it

The grid holds a melody written for this lesson with no chords yet. Tap the bottom row of a bar to cycle through C, F, G, Am and no chord, and listen to how each one changes the colour. Then press Suggest chords. For each bar it picks the chord that shares the most notes with the melody, giving extra weight to beat 1. Compare its answer with yours; the suggestion is a starting point, not the only right answer.