Stacking in thirds - significance of adding the 7th?

[Audiophage]

Sevenths make more musical sense when you go beyond diatonic harmony.

In Jazz everything has sevenths, you can't just go around playing triads....

 

[hleach]

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

But it's function may be ambiguous until that 7th is added....

Say an intro to a song goes: |D Em| x4. What key am I in and what are the chords' functions?

 

[EADG mx]

But it's function may be ambiguous until that 7th is added....

Say an intro to a song goes: |D Em| x4. What key am I in and what are the chords' functions?

Good point, but D7 - Em7 is ambiguous too, albeit to a lesser extent. Adding the 7th in this case just tells me we can't be in D major because a dom7 chord can't be the tonic. It still doesn't outright state that we're in G Ionian or any of it's modes (although that's a safe bet).

 

[Pacman]

Good point, but D7 - Em7 is ambiguous too, albeit to a lesser extent. Adding the 7th in this case just tells me we can't be in D major because a dom7 chord can't be the tonic. It still doesn't outright state that we're in G Ionian or any of it's modes (although that's a safe bet).

But DMaj7 to Em7 is fairly clear, isn't it?

 

[kingbee]

The 7th is used to add color and to ephasize movement. That tense tritone in the V7th chord (B and F in G7, for instance) wants to pull towards the neighboring chords tones of the I chord (C and E in C). You can hear this a bit better if you play through the cycle of fifths and try to end on a 7th chord. So if you play E7 A7 D7 G7 and let it end there, it sounds incomplete. You really need the C at the end to get the resolution. Now try the same sequence and stop at G instead of G7. You'll feel the same sense of rest.

There's lots to learn here with regard to tritone substitutions and the half step movement of the tritones when playing the cycle of fifths using 7ths.