Chords

A chord is a collection of three or more pitch classes that sound together. (When only two pitch classes sound together, it is simply an interval.) Chords in the tonal system are based on the three most stable pitch classes of the overtone series (fundamental plus the first 5 overtones) Notice that the resulting chord is composed of pitch classes that are each a third apart. Tonal harmony is based upon this principle of chords containing stacked thirds. This is also referred to as tertian harmony.

Since there are three pitch classes present, this is referred to more specifically as a triad. The lowest pitch class of a triad is called the root. The names of other notes of the triad are derived from their interval above the root: the third and the fifth.

A diatonic triad may be built on any scale degree by adding a third and a fifth above it:

Notice that although the triads all consist of stacked major and minor thirds, they may be in different combinations. For instance, the triads built on scale degrees 1, 4, and 5 consist of a M3 on the bottom and a m3 on the top. We call this a major triad. The triads built on scale degrees 6, 2, & 3 consist of a m3 on the bottom and a M3 on the top. We call this a minor triad. Both are stable chords because of the P5 interval formed by the combination of the two thirds.

The triad build on scale degree 7 is different. It consists of two minor thirds that result in a diminished 5th when combined. We call this type of triad a diminished triad. A fourth and more rare type of triad results when two Major thirds are combined, resulting in an augmented 5th. We call this type of triad a augmented triad. Because these triads contain the unstable intervals of a o5 and +5, these are unstable chords.

Seventh Chords

By adding another third on top of a triad, we create a more dissonant chord that includes the interval of a 7th). This type of chord is called a seventh chord. Like triads, the names of other notes of the seventh are derived from their interval above the root: the third and the fifth and the seventh.

Since four pitch classes are present there are more combinations of major and minor thirds possible. In tonal music, however, only certain combinations are used. The following chart lists the seventh chords that function in tonal music:

Seventh chords built upon augmented triads are extremely rare, as are minor triads with a major seventh and major or minor triads with diminished sevenths.

Harmonic Vocabulary

Pop/Jazz Chord Symbols

There are several common ways to refer to various chords. The most general method is used in popular and jazz music and involves refering to the pitch class that serves as the root of the chord. Alterations to the basic chord, or added tones such as sevenths are notated with additional symbols. This system of notating chords does not specify how the chord functions or relates to other chords, but is a very simple and easy way of specifying the quality of the chord. The following table lists this method:

Triads:

Additions/Alterations to Triads

Roman Numerals

Another way of specifying chords that reflects their relationships to a particular key uses roman numerals to indicate the scale degree upon which the chord is built instead of pitch names. In this method, the roman numeral is upper case for major (and augmented chords), lower case for minor, and lower case plus the small superscript "o" for diminished triads. However, in this method, the key of the music must always be specified in order to determine the exact pitch classes present in the chord. This method also uses a "7" to indicate seventh chords but does not need to specify the type of seventh, since it is assumed that the seventh will be whatever pitch class would normally occur within that key. (Note: the vii chord is a special exception to this and uses the "o" for fully diminished seventh chords and "ø" for half diminished seventh chordss.)

Notice that this method defines chords according to their relationship within a key, thus providing a means of discussing functional relationships of chord progressions. For this reason, this the prefered method of defining chords for the discussion of harmonic function in music theory.

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