There are five harmonic structures that occur within the tertian system of harmony that must be understood completely. These are the five basic seventh chords that are used most frequently in tonal music. They are as follows:
- Major seventh
- Dominant seventh
- minor seventh
- minor seventh with b5 (half-diminished)
- full diminished seventh
The first four of these structures are derived from the major scale and the last (full diminished seventh) is derived from harmonic minor. Let’s examine each type and find some simple formulas that will make it possible to easily determine the structure of these chords in any key.
First of all we need to know the major scale in all keys. This is not so daunting a task if you understand the familiar formula to create a major scale: whole-whole-half-whole-whole-whole-half in which “whole” represents a whole-step or the distance of two frets and “half” represents a half-step or the distance of one fret. It is then very easy to construct a major scale from any pitch; just remember to follow alphabetical order when naming the notes in order to correctly assign a sharp or flat to a note. We will use C major for the examples to keep it simple.
Since our tonal system of harmony is based on thirds (tertian) the chords can be derived from the scale by simply playing every other note. This is where the keyboard becomes a very handy tool for hearing and understanding the chords. Since the C major scale is made up of the white keys only, all we need to do is play every other key to make the chords. So let’s form the seventh chords in the key of C major.
- Chord I: C E G B
- Chord ii: D F A C
- Chord iii: E G B D
- Chord IV: F A C E
- Chord V: G B D F
- Chord vi: A C E G
- Chord vii: B D F A
Now what are the qualities of these seven chords? The quality is determined by the intervals that make up each chord. To really understand this we must look at the intervals between each tone of each chord. When we do we will find that there are four unique structures within the scale. The easiest way to show the interval structure is by numbers which show the number of half-steps between the chord tones. For example the distance from C to E is 4 meaning four half-steps. The distance from E to G is 3 meaning three half-steps. So the interval formula for chord one (C E G B) is 4-3-4 meaning there are four half-steps between C and E, three half-steps between E and G, and four half-steps between G and B. By definition this structure is known as a Major seventh chord because traditionally the chord is named by describing the quality of the triad (C E G) and the quality of the seventh (B). The triad (4-3) is major and the seventh is also major (the distance between C and B is eleven half-steps or a major seventh). We then say this structure is a Major (triad) Major seventh chord or simply a Major seventh chord. The structures found within a major scale then are as follows. Commit this list to memory as it is extremely important to know which chord types occur on which scale degrees when analyzing harmonic progressions.
- Chord I (C E G B) 4-3-4 = Major seventh
- Chord ii (D F A C) 3-4-3 = minor seventh
- Chord iii (E G B D) 3-4-3 = minor seventh
- Chord IV (F A C E) 4-3-4 = Major seventh
- Chord V (G B D F) 4-3-3 = Dominant seventh (Major/minor seventh)
- Chord vi (A C E G) 3-4-3 = minor seventh
- Chord vii (B D F A) 3-3-4 = minor seven b5 or half-diminished seventh
Notice that chords one (I) and four (IV) are the same type (Major seventh), and chords two (ii), three (iii) and six (vi) are the same type (minor seventh). Chord five (V) is unique (Dominant seventh) and chord seven (vii) is unique (minor seven b5). The unique quality of the five (V) chord is extremely helpful in figuring out key areas in jazz since this chord is usually the Dominant or V7 of the key we are in at the moment.
We still have one other structure to examine that does not occur naturally in a major scale but is derived from harmonic minor; that being the diminished seventh chord that is built on the raised seventh degree of the scale. In A minor the chord would be G# diminished seven and is spelled as follows:
G# B D F with an interval structure of 3-3-3 which again is different from the others and is the last of the five types.
Now for the simple formulas; the easiest way is to compare the five types and see how they differ from one another. This is the system that is used in most jazz texts. We start with the major seventh chord as the fundamental structure and lower certain chord tones until we end up with the diminished seventh chord at the end.
When we compare the major seventh chord (4-3-4) with the dominant seventh chord (4-3-3) it is clear that the only difference is that the distance between the last two tones (fifth and seventh) has been reduced by half-step. So to convert any major seventh chord to a dominant seventh we just lower the seventh of the chord by half-step:
- C E G B = C Major7
- C E G Bb = C Dominant 7 or C7
When we compare the dominant seventh chord (4-3-3) with a minor seventh chord (3-4-3) it turns out that the difference is the distance between the root and third and we can convert the dominant seventh chord to a minor seventh chord by just lowering the third by half-step:
- C E G Bb = C7
- C Eb G Bb = C minor7
When we compare the minor seventh chord (3-4-3) to a minor seven b5 or half-diminished seventh chord (3-3-4) we find that the difference is the distance between the third and fifth. So to convert a minor seventh chord to a minor seven b5 we just flat the fifth. (So that’s why it is called a minor7b5)
- C Eb G Bb = C minor7
- C Eb Gb Bb = C minor7b5
Finally when comparing a minor seven b5 chord (3-3-4) to a diminished seventh chord (3-3-3) we see that the difference is the interval between the fifth and seventh. By lowering the seventh of the minor seven b5 chord by half-step we can convert it to a diminished seventh chord.
- C Eb Gb Bb = C minor7b5
- C Eb Gb Bbb = C diminished7
To put another way, the five formulas can be summarized as shown below using the degrees of the major scale:
- Major seventh chord = Major scale degrees (1 3 5 7)
- Dominant seventh chord = Major scale degrees (1 3 5 b7)
- minor seventh chord = Major scale degrees (1 b3 5 b7)
- minor seven b5 chord = Major scale degrees (1 b3 b5 b7)
- diminished seventh chord = Major scale degrees (1 b3 b5 bb7)
To determine the notes that make up any of the five types of seventh chords just start with the major scale that corresponds to the root of the chord you are working with and plug in one of the five formulas. For example let’s spell a G Major7 chord. We take the G major scale (GABCDEF#G) and extract the 1 3 5 and 7 giving us a spelling of G B D F#. To convert to G7 simply flat the seventh: (G B D F). To convert to G minor7 just flat the third: (G Bb D F) and so on down the list. With a little practice you will be spelling chords easily in no time.
Extensions or embellishments of the basic seventh chords (adding 9ths, 11ths or 13ths) is just a matter of counting up to whatever number you are adding, always using the major scale as reference, and adding that note to the chord. Adding altered tones or notes that are not diatonic within the scale are indicated by including an accidental before the number, i.e., b9, #9, b5, #5 etc. For example C7#9#5 would be spelled C E G# Bb D# (one of my favorite jazz sounds).
I have included a chart of the basic seventh chord voicings for guitar with roots on the sixth, fifth, fourth, third and second strings in the order described above. These voicings are the ones most commonly used by guitarists and should give you more than enough options for quite some time.