Stacking in thirds - significance of adding the 7th?

[Pastafarian]

Not necessarily a bass question, but reading this thread - What comes next in this pattern? - reminded me to ask something I've wondered about.

Whenever I've read about modes and their corresponding chords (e.g. D Dorian corresponding to Dm7), the chord always includes the 7th. Never fully understood this, until one of the replies in the thread above mention that the 7th is a third above the 5th (obviously) - makes sense given that the rest of the chord is built in thirds too.

Thing I don't understand - if you build a chord from the triad, you've got a complete chord that includes (pardon the amateurish terminology) whether or not the third is major/minor and the fifth is perfect/diminished (can't remember any modes containing an augmented fifth interval). Why add another third?

Thanks in advance.

[Richard Lindsey]

Because it adds a color to the chord that wasn't there before. Which is the same reason you'd add a 9th or an 11th or a 13th.

[Asher S]

Originally Posted by Pastafarian

Why add another third?

The best way to answer this is for you to play a major triad, then add a major 7, then play the same triad with a minor 7th and listen to how the chord "flavors" change. Then try the same thing but with a minor triad to start. So, adding a 7th just expands the harmonic flavor of the chord.

[JTE]

Play a C triad. Then play a C7 chord.

Yeah, Dmin vs. Dmin7 may not be that big a difference, but when you get to the V chord, it's the 7th that MAKES it dominant, and has the pull back to the I. And 7th chords are common enough in most music that for learning purposes it's good to know your chords all the way to the 7th.

jte

[Pastafarian]

Right, makes sense - fully appreciate colouring and flavouring the chord with 7ths and 9ths and so forth; was more confused why there's emphasis on the 7th in mode theory. JTE's post makes sense in that respect.

Cheers all!

[EADG mx]

The 7th is emphasized in mode theory because modes cover all of the chord tones and extensions of a particular chord. The 7th is the last of the chord tones. That is the basic precept of chord/scale theory: chords imply a scale (or scales) and vice versa. Keep in mind that this is usually applied to jazz; there are other ways to look at modality and this certainly isn't what modes were historically used for.

Adding the 7th to the chord as stated adds a new colour, and provides more (often more powerful) options for voice-leading.

[mambo4]

the way I've learned it (here, have a grain of salt) in a jazz context

the most important tones in a Chord are the 3rd and the 7th.

the third tells you if the chord is major or minor,

the 7th tells you if it's stable (maj 7th) or unstable (b7th)

"stable" meaning the chord is not implying any harmonic motion, unstable meaning the chord wants to move, usually as if from V7 to I.

All those crazy jazz substitution chords are often rearrangements of every chord tone except the 3 and the 7. so the harmonic function is unchanged.

[EADG mx]

Originally Posted by mambo4

the 7th tells you if it's stable (maj 7th) or unstable (b7th)

"stable" meaning the chord is not implying any harmonic motion, unstable meaning the chord wants to move, usually as if from V7 to I.

While I agree with your post, would you say the b7th is unstable on every chord, or only on major triads?

To my knowledge, a b7th on a minor triad (a m7 chord) is perfectly stable.

[MonetBass]

Let's say you're in the key of C. A V7 (dominant 7th) would be spelled G-B-D-F. When you play a progression in that key (say I - ii - V7), your ear really wants that F to resolve to the E, and the B up to C to get back to I. The tension is also caused by the fact that the 3rd and 7th in the V7 chord (B and F) are a tritone apart. Tension -> resolution is what it's all about.

[mambo4]

Originally Posted by EADG mx

While I agree with your post, would you say the b7th is unstable on every chord, or only on major triads?

To my knowledge, a b7th on a minor triad (a m7 chord) is perfectly stable.

While a m7 chord may not have the tension of dom 7 chord, I think it still implies some motion vs a minor triad or a min/maj7 chord.

Of course this kind of assertion is empty without context...a lot will depend on what other chords are present.

[HaVIC5]

The prevailing theory is that the "function" of the seventh is to provide a dissonance with the basic triad and give it its character with regards to other chords in the chord progression. The idea therefore is that the seventh says whether the chord is "moving" or "staying", which is a little different than "stable" and "unstable" because major sevenths can sound very unstable and dissonant in their own way.

The reason for this somewhat lies within the classical usage of sevenths, and somewhat in modern usage. Traditionally, the minor seventh above the root of the chord, like in minor 7 or dominant seventh chords, has a strong desire to resolve down a half step to a tone in in the next triad, usually a V dominant seventh chord (if its a II-7) or a I chord (if its a V7). Because the minor seventh is a dissonance with the root, traditionally, it must resolve. In contemporary music, it doesn't need to resolve - the dissonance is accepted as "color" rather than being unstable - HOWEVER, there are still resolution patterns that are left over from classical theory, such as the desire for it to resolve down a half step. This is why people sometimes classify minor sevenths as "moving", they don't tend to sound very final. Jazz pianists traditionally won't voice a tonic minor chord with a minor seventh (no I-7) because of this - it doesn't sound like its resting enough for it to be tonic. There might be the tacit implication that the chord will then resolve down a fifth to IV7. Generally they'll play a I-6 or a I-(maj7) instead, which, although probably more dissonant, sound more like "home plate".

The theory of major sevenths follows similarly, except in the opposite direction. Because the major 7th is the leading tone in the key, in classical theory there is a strong need for that note to resolve up a half step to the root of the chord, instead of down a half step to the third of the next chord in the cycle of fifths. In this way, it is "staying", because the seventh's desired resolution is up a half step to the root of the same chord other than a note in another chord. It still can be plenty unstable as a dissonance (min(maj7) chords, maj7(#5) chords), but it means the chord doesn't necessarily have that subtle pull somewhere else.

Now, sixth chords used to be pretty common in popular styles because unlike the seventh, the sixth doesn't create a dissonance with the tonic. In traditional thinking, the dissonances are 2nds, 4ths (remember, traditionally, 4ths sound fine to our ears today), and 7ths, and everything else is consonant (basically). When you add the major 6th, you have a consonance with the root, making it both "stable" AND "staying". Frequently, in the 30's and 40's, the tonic minor chord was voiced as a minor 6 (minor triad plus major sixth), and the major chord was almost certainly a major 6 chord. As time progressed, however, people desired the color of the major 7th and minor 7th (and the subsequent extensions beyond the octave) more and more, and so 6th chords aren't used as often. In Brazilian and Afro-Cuban music, however, even today tonic major 7ths aren't used often.

[JimK]

Originally Posted by Pastafarian

Thing I don't understand - if you build a chord from the triad, you've got a complete chord that includes (pardon the amateurish terminology) whether or not the third is major/minor and the fifth is perfect/diminished (can't remember any modes containing an augmented fifth interval). Why add another third?

I wouldn't call a triad a "complete chord"...unless that's really the sound/color you want to hear.
The 3rd & 7th are most important (especially in tri-tone subs).

If I played guitar in a group setting, I would really consider voicing chords with no root...leave that to the bassist (or not).
There are guys (Steve Khan) that voice chords in an "open" fashion.
And there's a guy around town here that does that...makes playing bass an ease because any note I play works (mostly).

[JTE]

Originally Posted by JimK

I wouldn't call a triad a "complete chord"...unless that's really the sound/color you want to hear.
The 3rd & 7th are most important (especially in tri-tone subs).

Well, it's a pretty common stylistic thing too. For jazz-based stuff I agree completly, especially the part I snipped out of your quote about voicing when playing guitar.

But for lots of musics, the triad IS the sound they're going for. Folk music based forms are real fond of three-note chords. I had Pandora up yesterday and up popped Waylon Jennings doing Kristofferson's "Me and Bobby McGee". It reminded me of when I gave a buddy who was a really good jazz guitarrist some pointers for playing country music. We sat down with guitars, and the first thing he did when I suggested we start with "Good Hearted Woman" was to grab a D6/9 instead of a D major chord...

jte

[EADG mx]

Originally Posted by JimK

I wouldn't call a triad a "complete chord"...unless that's really the sound/color you want to hear.

Why not? Although definitely not in jazz, there's plenty of classical and pop music that makes exclusive use of triads.

[ryco]

The way it was explained to me:
The 3rd determines the quality of the chord (major or minor),
and the 7th determines the direction of the chord.

The Major seventh degree wants to resolve up a 1/2 step to return to the root.
The minor 7th usually wants to resolve somewhere else, causing chord movement.

[EADG mx]

A chord will still have the same function regardless of whether or not it has a diatonic 7th.

[JimK]

Originally Posted by EADG mx

Why not? Although definitely not in jazz, there's plenty of classical and pop music that makes exclusive use of triads.

I dunno...somewhere a long time ago, I thought it was taught 1-3-5 = a triad & 1-3-5-7 = a chord (9-11-13 were extensions).
Maybe it was in a Jazz Theory course...like I said, it's been awhile.

[Richard Lindsey]

Originally Posted by JimK

I dunno...somewhere a long time ago, I thought it was taught 1-3-5 = a triad & 1-3-5-7 = a chord (9-11-13 were extensions).
Maybe it was in a Jazz Theory course...like I said, it's been awhile.

Nope, a triad is the simplest chord. You don't need to have a 7th to have a chord.

[JTE]

A chord is defined as three or more notes played at the same time. Only two notes in an interval, but not a chord. Technically that means notes, note different pitches. So a "power chord" of just roots and fifths ain't a chord.

jte

[ahsbass6]

If you analyze Bach Chorales, Bach would double or even triple the root at a cadence, so the chord is implied. If you were hearing it played on an organ, the texture would be thin but because of the voicings used in the previous chords, it was understood what chord ( with the doubled or tripled root) was or implied. I say this because there is some excellent info posted here but as others have indicated tensions in a chord can imply harmonic movement. So when you discuss chords look at the phrase or progression. To Bach --- A power chord with just roots and 5ths could certainly be a chord. Just to add a little spin!