"Altered" thirds??

[alez]

They say that chords, in a traditional theory sense, are built from stacked thirds, which is why we use odd numbers for each of their notes (let's leave aside 6th, sus2, sus and whatever else). But the pretty common chord G7#9#5 (for example) has a perfect fourth between its 7th and 9th and a tone between its 5th and its 7th (being all 5, 7 and 9 odd numbers). Am I supposed to call those intervals augmented third and diminished third? Is that not kind of "cheating"? Like well, all chords are stacked thirds, so let's say that all these intervals are thirds by definition so that the definition stands?

I've just started to do some ear training for altered chords and my ear is pretty confused... and so is my brain!

[jazz oud]

The #9 and #5 aren't really #9 and #5. We spell it that way because it is convenient and it fits in with the "Stacked thirds" concept, but really that chord has both b3 and 3, and a b13 from the perspective of the key (since they are b7, 7 and b3, respectively, not #6, 7 and #2).
The b9 is the "real" ninth (being b6 of the key), and we omit the 5th and 11th.

So we have

R b3 3 (5 omitted) b7 (9 and 11 omitted) b13

The question is, why can we have both thirds? I don't know a good answer, except to say that the minor key has both b7 and 7 (b3 and 3 from the perspective of the V7), and people figured out it sounds good as long as the b3 is voiced above the 3 (or right next to one another for crunch).

I would say that the b3/3 interchange common in the blues had something to do with it, but you can find examples of it in classical music as well.

[alez]

That's a very interesting answer, as well as surprising and inspiring, thanks!

[alez]

Originally Posted by jazz oud

The question is, why can we have both thirds? I don't know a good answer, except to say that the minor key has both b7 and 7 (b3 and 3 from the perspective of the V7), and people figured out it sounds good as long as the b3 is voiced above the 3 (or right next to one another for crunch).

As we all know, there are other examples of minor and major thirds coexisting, with melodies using a minor third over I7 like in blues. Also in flamenco, some tonal subgenres (subgenres are called "palos" in flamenco) build their melodies off the phrygian scale with the harmonies built off a "major 3rd phrygian" scale and it's a very typical sound to hear both the minor and major 3rds at the same time during I.