Seventh chords are most easily thought of as an extension of triads, and our labeling system reflects this. In the same that we create a triad by inserting a 3rd into a 5th, we create a seventh chord by adding a 3rd on top of a triad. They are called seventh chords becuase this new note creates an interval of a 7th between the chordal root and the new pitch.
Because they are stacked thirds, seventh chords are still considered tertian harmony. They are prevalent in almost all styles of Western music, and we have developed many systems to describe how they function harmonically. Again though, we must first be able to classify and label them in an structural manner that does not rely on key-based functions. We name the chord members by the distance above the bottom pitch when the chord is stacked in thirds:
All seventh chords have exactly four notes, although certain chord members can occasionally be omitted (and therefore implied) depending on the context. With four pitches, there are four possible configurations that depend on which note of the triad is in the lowest voice. Like triads, we call these inversions, and we use the same shorthand system to label inversions that we used with triads.
Using the examples below:
Because ABC notation is not capable of using superscript, the inversion figures in the next example are notated as fractions. If you were to write these by hand or use custom notation software, you would notate all inversion figures using superscript. For example, a dominant seventh chord would be written as Mm7
A seventh chord is a diatonic chord containing four pitches, each separated by a 3rd.
There are 5 types of seventh chords, and we will be using both of the common terms for them:
MM
) or major seventh chordMm
) or dominant seventh chordmm
) or minor seventh chorddm
) or *half-diminished seventh chorddd
) or fully-diminished seventh chordThese terms are interchangeable but they are typically used in different circles. We will refer to them as the pedagogical name (e.g. major major, etc.) and common name (e.g. major seventh chord, etc.).
The pedagogical names are useful in illustrating the structure of a seventh chord and is unsurprisingly often used by theory teachers. As a student explained, “The first word is the triad quality (e.g. major triad on bottom), and the second word describes the interval between the root and the 7th.” This explanation highlights the mixed nature of this pedagogical categorization–it mixes a chord quality with an interval quality. The first word always describes the triad (the bottom three pitches) while the second word describes the interval quality between the root and the seventh chordal member. This interval is always a 7th.
Building on your knowledge of inversion figures from triads, it should be simple to derive the seventh chord system; it also uses the intervals above the bass, not the root of the chord.
As a reminder, we will be using the term inversion figure to discuss this shorthand method of identifying inversions. Other systems refer to these same superscript numbers as figured bass, bass position Symbols, or figures, but we need not argue about which name is better. As long as the student understands the difference between inversion figures, true figured bass (i.e. Baroque system for writing keyboard harmonies), and the shorthand used in leadsheet notation, it does not actually matter which term is used.