Chord

A melody rarely appears on its own. Almost always it comes with groups of notes that sound together and clothe it, like a garment. Those groups of notes are chords (Lacerda 1966, ch. XXV).

A chord is the simultaneous combination of three or more different sounds (Lacerda 1966, ch. XXV). With two sounds what you have is an interval; the chord begins with the third sound (Med 1996, ch. XL).

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C and E together are a harmonic interval, a third. Add the G, and they become a chord.

Melody and harmony

The difference between melody and chord is a matter of direction. Melody unfolds horizontally, one note after another; harmony follows vertically, one chord after another, each with its notes sounding together (Lacerda 1966, ch. XXV). Together they make a kind of fabric, with the melody running along and the chords marking the ground it runs over.

The melody need not be on top. It can appear in the middle of the chords or beneath them, in the bass (Lacerda 1966, ch. XXV). And when two or more independent melodies sound at the same time, one no longer speaks of melody with chords but of counterpoint (Med 1996, ch. XL).

Harmony, as a discipline, is the study of chords and of the way one leads into the next (Lacerda 1966, ch. XXV).

Different sounds: doublings do not count

The definition says different sounds. A C major chord played with two Cs, one low and one high, is still a chord of three sounds: C, E and G. Doubling a note, at the same pitch or in another octave, does not change the chord's classification (Med 1996, ch. XL).

That is why a guitarist strumming six strings may be playing exactly the same chord as a pianist using three fingers. Repeated notes add body and colour, but the chord is defined by its different notes.

The model chord and its shapes

Books present chords in a standard shape: the notes stacked as close as possible, from the lowest. Lacerda calls that shape the model chord. In practice, though, chords are used in all sorts of positions, with one or more notes doubled or tripled (Lacerda 1966, ch. XXV).

Here is the model C major chord and three of the many shapes it can take in music:

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1. the model chord, C, E, G, close together;
2. the notes spread over two octaves, with C doubled;
3. E in the bass, which is an inversion;
4. five notes, with G and C doubled.

All four are the same chord. The ear recognises C major in every one, even though each sounds different. Model chords come in a limited number and are formed by rules; the shapes they take are practically endless (Lacerda 1966, ch. XXV).

A chord can also be played note by note, as an arpeggio, and it is still the same chord (Med 1996, ch. XL).

Built in thirds

In the classical conception, chords are formed by stacking thirds. In that original shape the lowest note is the root, the one that generates the chord and gives it its name, and the others are named by the interval they form with it, the third and the fifth. That is why a chord is built and read from the bottom up (Med 1996, ch. XL).

This way of thinking has an author. In the eighteenth century the French composer and theorist Jean-Philippe Rameau proposed the triad in thirds as the basis of harmony, and much of the vocabulary of harmonic analysis still comes from him (Blatter 2007, part 3, art. 2).

Med names chords in thirds after the interval between the lowest and highest notes of the original shape (Med 1996, ch. XL):

Notes	Name in Med	Interval between the outer notes
3	chord of the fifth	fifth
4	chord of the seventh	seventh
5	chord of the ninth	ninth
6	chord of the eleventh	eleventh

The chord of the fifth, the three-note one, is what is also called a triad, and its four qualities have a topic of their own.

And beyond thirds

The third is not the only possibility. Chords can also be built from seconds or fourths (Med 1996, ch. XL), and composers, above all in the twentieth century, have used triads of fourths, fifths, seconds and sevenths (Blatter 2007, part 3, art. 2). A chord of fourths and a chord of seconds:

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Even so, the chord in thirds has been so dominant in Western music since before the eighteenth century that when a musician says "triad" without qualification, they always mean a triad in thirds (Blatter 2007, part 3, art. 2).

What comes next

The three-note chord in thirds, the triad, is the starting point: four qualities, major, minor, augmented and diminished, each with its own sound. One more third brings seventh chords, and the way one chord leads into the next is where harmony begins.