theoretical question

[larryb]

In, say, the key of C major, why does A7 work when playing CM7, A7, Dm7, G7? Sounds good, but theoretically why does it work when the A7 contains a C#?

[stanron]

Originally Posted by larryb

In, say, the key of C major, why does A7 work when playing CM7, A7, Dm7, G7? Sounds good, but theoretically why does it work when the A7 contains a C#?

A C note in Cm7 goes up to C# in A7 up to D in Dm7 and G7, whilst a B flat in Cm7 goes down to A and then G in A7 to F in Dm7 and G. Kind of like a bit of counterpoint.

[JonPR]

Originally Posted by larryb

In, say, the key of C major, why does A7 work when playing CM7, A7, Dm7, G7? Sounds good, but theoretically why does it work when the A7 contains a C#?

I think you mean Cmaj7, yes, not Cm7?

Anyway, stanron has the answer: the C# in A7 forms a "leading tone" up to the D in Dm7. That's it's classical purpose, but in jazz it will also go down to the C (which goes on down to B in G7).

A7 (or just A major) is known as a "secondary dominant": "V/ii", or V of the ii chord (Dm). Any key can have 5 secondary dominants, along with the primary dominant (G in key of C).
http://en.wikipedia.org/wiki/Secondary_dominant

[mr. beaumont]

The thing to take away here is that music need not stay strictly diatonic.

I'd argue the good stuff rarely does.

[ljguitar]

Originally Posted by larryb

In, say, the key of C major, why does A7 work when playing CM7, A7, Dm7, G7? Sounds good, but theoretically why does it work when the A7 contains a C#?

Hi larryb...

My question is "Why does it have to work theoretically?"

First came music, and then notation was created so we could recall it and pass it on (perhaps the earliest recording method), and then theory was created to attempt to explain it.

If it sounds good, I don't require a theoretical treatise to defend it, nor describe it.

[Doug Young]

I'm with Larry on the "sound came first", but the theory answer to why is because the A7 is the V7 of D, so you're temporarily modulating to Dm for the A7-Dm sequence. But then the Dm is the ii of C, so you're right back in C. Music doesn't have to stay entirely inside diatonic chords, and all but the simplest tunes usually do more interesting things.

[JonPR]

Originally Posted by ljguitar

Hi larryb...

My question is "Why does it have to work theoretically?"

First came music, and then notation was created so we could recall it and pass it on (perhaps the earliest recording method), and then theory was created to attempt to explain it.

If it sounds good, I don't require a theoretical treatise to defend it, nor describe it.

Well, you need theoretical terms (not a treatise) to describe it, but yes, that's all that theory does: describe, not explain.

Even chord symbols are theoretical terms, and where would be here if we didn't know what to call the shapes our fingers make on the fretboard?

A7 going to Dm works. Clearly. That's really all we need to know.

Curiosity about what such a usage might be called (the theoretical name for it) is all very well, as long as one doesn't feel the answer explains anything.
"Secondary dominant" is a piece of jargon, that's all.
Knowing that phrase simply means that you know that the practice is not unusual, that it's been used often enough before for someone to have invented a name for it. So it's not strange or weird, and it's certainly not "wrong" (if those things worry you, and of course they shouldn't).
It doesn't tell you why it works. No theory will tell you that.

The answer to how it works, OTOH, lies in something which is easy to observe (with no theory knowledge), if we look at (and listen to) how the chords move. We can see (and hear) how that C# on the A7 chord moves to a D (or C) on the Dm7 chord. We can see and hear how different it is from a C on an Am7. There's more sensation of "tension" or "forward motion" (or however one wants to describe the subjective sensation) with A7 and its C#.
We can then decide whether we prefer one over the other, or rather in what circumstances we might prefer one over the other.
That's all we need to know as musicians (players, improvisers, composers).
The jargon is an optional extra. The only time one might need theory to "defend" something is when composing for an academic exercise.