What is up with the 7th Chord?

7th chords are very common in blues.

A7, D7 and E7 chords for example, are the staple diet of a blues rhythm guitarist. So much so that if they say "A" they probably mean "A7". Jazz musicians do this too.

In practice when building one of these chords on guitar, e.g. A7, we replace the 8th note, the A, with a G... which is a flattened 7 in A major. So that's just another hint that musical theory was written by drunks.

(The 8th note, the A, still "belongs" in the chord, but we sacrifice it, omit it, because we need to use that string to play the G. Only have so many fingers on your hands, you know.)

So I looked up "how to build a 7th chord"

And found this short instructional video....

So the chord with the flattened 7th is a "dominant" 7th (not a "Major 7", which would've had the non-flattened 7th). So "A7" (pronounced "A" by some blues rhythm guitarists) is A dom 7. It's also known as a "Major Minor 7th chord" which is perfectly COIK.

...then he goes through the 8 different kinds of 7th chord....

Not that you're going to use all of those. At least not in one song. At least not on your first day. (Welcome to 7th Club).

Any chord may also have a "sus" variation, what this means (e.g. sus2 or sus4)...

And all of that was interesting to me -- but here's what I really want to say about 7th chords.

In a lead sheet it may have the chords listed, and they often write in a kind of shorthand.

If it says "A—" (A followed by a significant dash) then it means "A minor". If it says "A" it may mean A7. That's what gets me.

And then there's the voicings, i.e. which particular arrangement of keys are used to create that chord. For an A7, here is what my jazz fiend buddy says:

Jazz fiend: For an A [meaning A7] you can leave out the 5th, that's implied anyway

Me: Oh so we leave out... in this case, the E? Because that's contained in the overtones of the root note, the A, right?

Jazz fiend: Yeh, and add the 7th

Me: The G? Okay so we've got an A and a G, what else?

Jazz fiend: Well, now leave out the A, because that's implied anyway.

Me: leave out the A? In an A?

Jazz fiend: yeh. Strip it back.

Me: It's just... it's just a G? Is that even a chord?

Jazz fiend: sure it is. Depends what you hear.

Me: Uhhh, okay.

Jazz fiend: and maybe add this:

(he adds in a C, two octaves up)

Me: so (thinking) that's a... minor... a minor 3rd note?

Fiend: Or this

(he drops the note by half a semi tone)

Me: that's... that's just wrong?

Fiend: yeh it's a better chord. Okay. Maybe leave out the G.

Me: (silence)

I have not spoken since.