A chord is three or more notes sounded at the same time. Play one note and you hear a pitch; play three of the right ones together and you hear a colour, bright or dark, settled or restless.
On a keyboard the pattern of keys repeats every twelve keys, white and black together. The distance from one key to the very next one, black or white, is a semitone, the smallest step in Western music. Twelve semitones make an octave: the note at the top shares its letter name with the one at the bottom and sounds like a higher copy of it. Counting semitones is all the arithmetic this lesson needs.
The triad: root, third, fifth
The simplest chord is the triad. Start on any note: that is the root. Count up to the third note of its scale, then to the fifth. Those three notes, played together, are the chord named after the root.
In semitones it is even simpler. From C, count four keys up and you reach E: that is the third. Count seven keys up from C and you reach G: that is the fifth. C, E and G make C major. The same counting works from any root, four up and seven up. From G it gives G, B and D; from F it gives F, A and C.
Major and minor
Lower the middle note, the third, by one semitone and the chord turns from major to minor. Same root, same fifth, a different mood: C, E and G become C, E♭ and G. A major triad is the root, four semitones up and seven up. A minor triad is the root, three up and seven up.
| Chord | Root | Third | Fifth | Semitones up |
|---|---|---|---|---|
| C major | C | E | G | 4 · 7 |
| C minor | C | E♭ | G | 3 · 7 |
| G major | G | B | D | 4 · 7 |
| F major | F | A | C | 4 · 7 |
| A minor | A | C | E | 3 · 7 |
| D minor | D | F | A | 3 · 7 |
These two shapes are where nearly every chord you meet later begins: sevenths and the other richer chords are triads with notes added on top.
On a guitar, the open C major shape gives you the same chord. From the fifth string to the first: third fret, second fret, open, first fret, open, with the low sixth string left silent. That sounds C, E, G, C and E: three letters, two of them doubled. The open A minor shape, from the fifth string to the first (open, second fret, second fret, first fret, open), sounds A, E, A, C and E: root, fifth, root, third, fifth.
Doubling a note, or playing it an octave higher, does not change the chord's name. What counts is which letters are present.
What comes next
Build a triad on each note of the C major scale, using only the white keys, and you get seven chords: C major, D minor, E minor, F major, G major, A minor and B diminished. The last one stacks two minor thirds (B, D and F), so its fifth is squeezed to six semitones; it sounds tense, and it gets a lesson of its own later.
Three of the seven, C, F and G, already make a working accompaniment in the key of C. Play them in that order and come back to C.
One more test of the idea. A two-note "power chord", the root and the fifth with no third, is neither major nor minor: the third is the note that decides.