chord alterations to popular progressions

Question

Using the jazz progression (iii7 - vi7 - ii7 - V7 - I7) in C Major, we have (em7 - am7 - dm7 - G7 - CMaj7).

But... I often see this progression wrote or played as: (em7 - A7 - dm7 - G7 - CMaj7)

So, the initial vi (am7) has apparently been changed to (A7)..... aka the minor 7 chord has been replaced with a Dominant 7 chord

I have questions about all of this:

1. Why is this a common change?

2. Why... if a Dominant 7 is to be played, am I seeing it wrote down originally as (vi)? If the vi, in this case the chord of am, is never played, why even mention it?

3. What is the name of this technique in general, so that I can study it further?

4. In this certain progression, what is so special about changing the (vi) chord & not the others? Why isn't the initial (iii) chord of em changed to E7?

Answer 1

Why is this a common change?

By departing briefly from the home key, some color is added to the phrase. It's the twist of lime added to your otherwise ordinary gin & tonic, so to speak.

Why... if a Dominant 7 is to be played, am I seeing it wrote down originally as (vi)? If the vi, in this case the chord of am, is never played, why even mention it?

Jazz often looks at chord progressions in terms of the root movement, dealing with the actual chord quality more loosely. Sufficiently skilled musicians can hear both that the roots are moving in a ii-V-type pattern (i.e., by descending fifths) and the qualities of each chord.

What is the name of this technique in general, so that I can study it further?

The "dominant vi" chord is called a secondary dominant. For example, the A7 chord is your example is the dominant chord relative to the following D chord. Searching "secondary dominant" on this site will produce many related posts.

In this certain progression, what is so special about changing the (vi) chord & not the others? Why isn't the initial (iii) chord of em changed to E7?

In theory, any of the minor-7 chords could be changed to dominant seventh chords — i.e., secondary dominants. But the more this is done, the further away from the underlying flavor of the main key one gets. Using iii7 for the first chord, for example, keeps the ear in the main key, but using III7 pulls the ear toward the key of VI.

Answer 2

It's the 'cycle of 5ths' bass line that matters. On top of that you can keep diatonic, throw in the odd explicit 'secondary dominant' or make them all dom7 shape chords. Here's a very well-known example using all dom7ths.

And it's all in F major. No need to explain it away as temporary tonicizations or 'borrowed' chords. It's VI7, II7, V7, I in F major. That's OK.

I can't think of a song that does either of the following versions of a 'cycle of 5ths' progression off-hand, but it would be quite possible. Notice my heretical chord label in the bottom line. But I stand by it. That C♮ is no way a SHARPENED note!

As to why jazz musicians are sometimes sloppy over chord labelling? Perhaps because in their world, EVERYTHING is up for alteration or substitution. Show a jazzer a simple ii7, V7, I progression - Dm7, G7, C - they won't stop to ask permission before changing it to D7, G7, C or even Dm7, D♭maj7, Cmaj7.

Answer 3

https://en.wikipedia.org/wiki/Ragtime_progression
There's no reason they all couldn't be dominant chords. It's a sequence of falling fifths, so choosing which chords to make secondary dominants is up to the songwriter.

Also

em7 - A7 - dm7 - G7 - CMaj7

Happens to be two ii-V-I's superimposed, which jazzers like. Like Aaron said, it doesn't matter that the first I (dm7) is actually an i. The chord qualities of a ii-V-I are usually adjusted for the musical context.