Different types of 7th chords

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Different types of 7th chords

Postby Bob Lewis » Sun May 31, 2009 9:16 am

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I got a response back that said that for instance a C seventh chord,
c, e, g, and b flat, is different from a major seventh chord, which is c, e,
g, and B natural. Well that was news to me, and so what do you actually term
these seventh chords in music theory, the ones we play on autoharp, since I
now know that they're neither major nor minor?

<>

[Bob]

Relative to other types, they are just "seventh" chords with no qualifier. However, it does depend on the context, the relationship of the chord root note to the key signature. "Root" for C7 in the key of C is the C note, for example. As an example of a 7th chord, the V7 is the "dominant 7th". The I7 is the "flatted 7th" or "tonic 7th" in terms of scale degree like the "dominant". There is a name for each step of the 7 tone, diatonic scale, and "dominant" is one of them.

For example, in the key of C, no sharps or flats, the G7 chord (GBDF) is the dominant 7th or V7 relative to the C root and key signature, while the C7 (or I7 or tonic 7th) requires a Bb note that is not in the C scale (CEGBb). It can rightfully be referred to as a "flatted 7th". We see from the multi-key autoharp layout that the C7 chord is borrowed from the neighboring key of F, which has that Bb note. We thus observe that the I7 is not diatonic, while the dominant 7th is diatonic, the only 7th chord that is a Major third plus two successive minor third intervals. There is only one (unqualified) "7th" enabled by a diatonic scale. That is the case, because the diatonic scale has a half step interval between the fa-sol and ti-do or 4-5 and 7-1. The various possible combinations of major and minor third intervals within a single diatonic scale present us with only one "7th", that rooted at the V, sol, or dominant note.

Parenthetically, note that conventional Western music theory is in reference to the Ionian mode of the diatonic scale, wherein the 7 scale notes (per octave) are referred to as in order beginning with the key root. In C, for example, that would be the C note. That sequence is also known as the "major scale".

There are both "Major 7ths" and "minor 7ths" that are diatonic but only this one M+m+m 7th. The uppercase M indicates a major third interval and the lowercase the minor third. The Major 7th chord is M+m+M, and the minor 7th is m+M+m. With the three possible types then, you have 7th, M7th, and m7th; "seventh", Major 7th", and "minor 7th". The "diminished 7th" is m+m+m. The "augmented 7th" is M+M+m. These last two are not possible within a diatonic scale, i.e. they are chromatic. The diatonic scale allows the "diminished" m+m, rooted on the 7 tone or ti, but not with the added 7th tone.

All 7th chords are a fourth tone added to a base triad, so the names are based in the identity of the underlying triad plus whether the 7th tone is a Major or minor third above the triad. Where the naming system is flawed to the point of being near useless, certainly confusing and thus this overall question, is that there is a Major-minor 7th, a Major-Major 7th, and a minor-minor 7th, yet they are called "7th", "Major 7th", and "minor 7th" respectively. It gets more complicated when you move to a chromatic scale. What you can see though is that duplicate occurrences of the word "Major" or "minor" can be omitted and still have three unique names. When we ordinarily refer to a "Major 7th", for example, that really translates to "Major-Major 7th", with "Major-minor 7th" having been reduced to just "7th".

With the autoharp, I don't favor the use of scale degree names like "dominant 7th". Since the autoharp is so graphic and relatively logical in terms of chord relationships, I favor use of numbers, Roman numerals in particular. Because of relatively consistent chord layout patterns from key to key, it seems useful to speak of chords in generic terms, since transposing to another key is relatively straightforward, if not too distant a relative key. The most often encountered or most useful, I believe, are the I7, V7, II7 (V of V), VI7, and III7, all relative to the root note of a key signature and particular diatonic scale. For the key of C, for example, those would correspond to C7, G7, D7, A7, and E7 respectively. Looking at ones autoharp, these are likely to be lined up all or mostly in a row and in that same order. In that general context, they are just "seventh" chords, all of them M+m+m. A standard autoharp only has that single type of 7th chord, M+m+m.

The scale degrees for a diatonic scale are tonic, supertonic, mediant, subdominant, dominant, submediant, and leading tone, corresponding to 1, 2, 3, 4, 5, 6, 7; or I, II, III, IV, V, VI, VII; or do, re, mi, fa, sol, la, ti.

Note that with the diatonic scale alone, the Major 7th chord is one of the very few possibilities that includes two notes only a half step apart, the ti-do or fa-sol, so it is relatively dissonant, certainly demanding resolution of that 7th sound, always "leading". The diatonic scale supports both a IM7 and a IVM7. In C, for example, those would be CM7 (CEGB) and FM7 (FACE) respectively.

That should be enough for the question to be satisfied. There are good websites for researching such things. Check the Hot Links section of my website for lots of good study on music theory relevant to understanding the autoharp and music in general. An example that clearly illustrates the basics is http://www.danmansmusic.com/chords.htm.

Here are the common 7ths, stated as relative to the C diatonic scale and key signature:

M+m+m - (Major-minor) "7th" (the common "dominant 7th" relative to a key) (e.g. GBDF is G7)
M+m+M - (Major) "Major 7th" (e.g. FACE is FM7)
m+M+m - (Minor) "minor 7th" (e.g. ACEG Am7)

A couple more combinations are strictly chromatic:

m+M+M - minor-Major 7th (e.g. CEbGB is CmM7)
M+M+m - augmented minor 7th (e.g. CEG#B is C+m7 or just C+7 since there can be no +M7)
m+m+M - diminished Major 7th aka "half diminished" (e.g. CEbGbBb)
m+m+m - diminished (minor) 7th (e.g. CEbGbBbb or, in terms of autoharp string markings, CD#F#A)

Unusual about the augmented minor 7th, M+M+m, is that it has the ti-do interval of the Major 7th, but the 7th, as with other types, is actually named for the interval that is added to the underlying triad. Adding a Major third interval to an augmented triad would come back to the root, so there is no such combination of M+M+M. Only the minor interval is added as M+M+m.

Also note that the word "minor" is dropped from "diminished minor 7th", since that is the more common of only two possible types of diminished 7th. The "minor" is implied, while the [diminished Major 7th] explicitly describes the exceptional 7th interval type, known in practice as a "half diminished".


Bob Lewis ♫
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