Fig 1. The chord of C major as played on a piano.
Fig 2. Harmonized C major scale.
Fig 3. Harmonized chords of C major played on guitar (with some doubling of notes).
Fig 4. Chord types by interval.
Fig 5. Types of chord.
A chord is a set of two, three, or more notes, usually spaced at harmonious intervals, that are played simultaneously. When the notes making up a chord are played sequentially rather than simultaneously, the result is known as an arpeggio. A slang term for a chord is a change. A chord progression or chord sequence is a series of chords played in succession.
The most commonly encountered chords are the major and minor triads, and the augmented and diminished triads. Chords are named after their root note, or tonic. For example, the chord of C major is made up of the notes C, E (the major third) and G (the fifth). A chord with just two notes is called a diad; a particularly effective form is the power chord (root plus fifth). A chord with four notes is a tetrad. More complex chords are possible but not encountered much outside of certain musical genres such as jazz and fusion.
Chords are one of the basic elements of harmony – in other words, the playing together of notes that sound harmonious or pleasing to the ear. The system of chords comes about through the harmonization of scales. This means simply the playing simultaneously of notes from a scale that sound good together.
A bit of history
Originally, music was written one line at a time, usually for singers or small instrumental groups. When music evolved to have more than one line sung or played at the same time, people found that certain notes sounded good together, and other notes didn't. As a result, rules were created that stated what intervals could be played harmoniously together – for example, thirds, fifths, and sixths. To begin with, composers didn't think of chords; what they did think about were individual lines of melody that intersected to form what we know today as chords. It wasn't until Jean-Philippe Rameau theorized about chords in the eighteenth century that chords were named and understood. But as with a lot of theory, Rameau spoke about chords after they happened, so all he did was name something that already existed.
Rameau spoke of groupings called triads – three-note chords built on fifths. He theorized that triads are formed by stacking or harmonizing a major scale. And what he thought back in 1722 still holds true today.
The harmonized major scale
Chords and harmony come from a harmonized major scale. The easiest way to understand the process of constructing chords is to start with the simplest and most common of scales – the major scale. Consider the notes of the C major scale. These are simply the white notes on the piano, beginning on C:
The step-pattern, or sequence of intervals, between the notes of a major scale is:
tone tone semitone tone tone tone semitone
The first of the harmonized chords of the C major scale is built from the first (C), third (E), and fifth (G) notes.
The gap between each of these notes is a third. The gap between the first two is a major third (two tones) which defines this to be a major chord – the chord of C major.
To get the next chord we take D as the starting note and add in the next two thirds: F and A.
The gap between the first two notes this time is only a tone and a half – a minor third. So this is a minor chord – the chord of D minor.
The next chord starts on E and includes the two thirds above this: B and G.
Again there's a minor third between the first two notes so this is a minor chord – E minor.
Moving up to the F:
This time the first interval is a major third, so the chord is F major.
To construct the next chord in the harmonized C major scale, it's easier if we add another octave:
This is G major. Next up:
Which is A minor (because of the minor third between the A and C.
The final chord in the harmonized C major scale is an oddball.
Its root note is B but the chord is neither B major nor B minor. Both intervals above the root are a minor third, which makes this a diminished chord – B diminished.
So now we have the full set of triads based on the harmonized scale of C major (Fig 2):
C major D minor E minor F major G major A minor B diminished C major
These can be played as full chords (with some notes appearing twice, one or two octaves apart) on the guitar in open position, as shown in Fig 3.
The principle is the same for finding any major triad. All we have to do is take the first, third, and fifth notes of the major scale. For example, to find an E major chord, write out the notes of the E major scale (E, F♯, G♯, A, B, C♯, D♯, E) and select the first (E), third (G♯), and the fifth (B).
To form a minor chord, the third of the corresponding major chord has to be lowered by a half-step (semitone) – equivalent to one key (black or white) on the piano or one fret on the guitar. For example, the notes for an E major triad are E, G♯, B. So, following the rule just stated, an E minor triad is E, G, B.
The other triads
Classical music, jazz, and popular music contain diminished chords. Blues music rarely uses diminished chords because they aren't part of the standard 12-bar blues progression; however, some rock music does use diminished chords (Fig 4).
To make a diminished triad lower both the third and the fifth of the corresponding major triad by a half-note. So, for example, the notes for E diminished are E, G, B♭. The triad formed from the seventh note of a major scale is diminished. In the case of the C major scale described above, the triad form with B as the root note is B diminished.
The final triad is called an augmented triad. These triads crop up in jazz and classical, but are pretty rare in rock, although the likes of Frank Zappa and Steve Vai have used them. An augmented triad is a major triad with an augmented fifth. For example, to get C augmented, start with the basic C major triad (C, E, G), and raise the fifth G up one half-step to G♯, to give the C augmented triad C, E, G♯. Augmented chords don't occur naturally in the harmonized major scale. Nonetheless, they're important to understand.
A seventh chord, as the name suggests, is built from a triad by adding on the seventh note of the scale. There are several varieties, distinguished by both the type of triad and the type of seventh used. The most common are:
Extended chords (or chord extensions, if you prefer) are tertian chords (built entirely from thirds) that contain notes that are more than an octave above the normal triads (Fig 5). (For this reason, seventh chords aren't considered to be extended.) Ninth, eleventh, and thirteenth chords are extended chords. The additional notes are dissonant (see dissonance) to the basic chord. For more on this subject, see the separate article on extended chords.
Added tone chords
An added tone chord is a non-tertian chord composed of a tertian triad and an extra "added" note. The added note is not a seventh (three thirds from the chord root), but typically a non-tertian note, which cannot be defined by a sequence of thirds from the root, such as the added sixth or fourth. For example, the added tone chord C6 is composed of the C major triad C, E, G, plus the sixth note of the scale, A.
Roman numeral chord notation
Roman numerals can be used to indicate major and minor chords.
Major chord: I, II, III, etc.
Minor chord: i, ii, iii, etc.
Augmented chord: I+, II+, III+, etc.
Diminished chord: vi°, vii°, etc.
Half-diminished chord: viiø7, etc.
Extended chords: ii7, V9, V13, etc.
Altered tones or chords: #iv, ii#7
In this notation, the harmonized triads of the major scale, for example. may be written: I, ii, iii, IV, V, vi, vii°.