A triad is a three-note chord; this much you know. Triads take care of most of the basic harmony, but not all of it! You build all of your chords in thirds. When you derived diatonic harmony from major and minor scales, you simply stacked thirds from each root and came up with seven triads — all built in thirds. Your knowledge of intervals allowed you to decode what the triads were and what order they appear in.
Now, what would happen if you added another third to your triads? Well, simply, you'd form seventh chords, which are named so because the last interval you add is seven notes away from the root. You could call them quad-ads, but it doesn't have the same ring to it as “seventh chord” does — plus it sounds like a form of sit-ups!
The majority of the harmony you deal with is built in intervals of thirds.
This type of harmony is called tertian harmony and is the basis for “common practice” or “tonal” harmony that is studied and still utilized today.
Remember the term
Now, to make them into seventh chords, you simply need to add another third on top of each triad, adding an interval of a seventh if you measure from the root. Remember to use only the notes from the F scale (F–G–A–Bb–C–D– E–F) when adding your thirds to keep this example diatonic. Doing so will leave you with these seventh chords:
From this list of chords, you see four different types of seventh chords: major seventh, minor seventh, dominant seventh, and half diminished seventh.
Notice that in FIGURE 9.2, the Roman numerals used to analyze the chords did not identify which kind of seventh chord you were encountering. They simply added “7” next to each Roman numeral. The answer had to be spelled out. Because of this, you need to understand what makes a major seventh different from a dominant seventh, even though both use uppercase Roman numerals (signifying major chords) with sevenths attached to them.
Point to Consider
Typically, even if your experience with theory is limited, you've come across, played, or heard about a G7 chord (or any other root). That is one kind of seventh chord (as you will learn about). This book uses the term seventh chord as a broad category. Seventh chords come in many different types, and G7 is simply one type.
Diatonic seventh chords are definitely useful, and in Chapter 10, you will see just how useful they are, when the focus is on chord progressions. For now, you need to understand the structure of the seventh chords, because there are more than just the four diatonic seventh chords used in modern music!